Merman Kiri AU
Kiri Baku
Eventual nsfw, mostly pining and fluff... I'll tag when I get to it

For Chey https://twitter.com/theGingerTrekie/status/1249076806059274240

A long time ago mermaids were very social creatures, travelling in packs across the ocean, their hair would appear as a glittering rainbow of colors below the surface of the water. Mermaids were known for using their beauty and magical voices to lure sailors to their deaths.

But times have changed. Humans learned how to capture and use mermaids for their own selfish needs, exploiting their magic and killing them for their scales. Mermaids were forced to become solidarity creatures, their colorful hair drew too much attention when traveling together.

They would explore the world by themselves, only meeting once every 5 or so years to mate. 

Kirishima Eijiro was young for a mermaid, only about 50 years old. His parents raised him for the first couple years of his life, taught him how to hunt and stay safe.

He had never seen a human before, only huge metal ships that spit out dark clouds and occasionally drop strange objects into the water. Eijiro avoided these human thighs as all costs, but he has always been curious.

His mother used to be a very powerful and feared mermaid, hunting humans because it was "almost too easy". She would tell Eijiro stories of the humans she had taken, wistfully recounting the handsome men she would "play with".

Eijiro needed to see a human, just once, then he could move on with his life. He had been slowly venturing closer and closer to a beach where humans visited. The beach was never very crowded, Eijiro would only see 10 or so humans during the day, but he was still nervous.

From far away they looked weird, they had two extra bigger arms instead of fins and a tail and they carried around big bags and boards with them. They were noisy too, Eijiro only poked his eyes out of the water to watch them because it was too loud.

He stayed beyond the reef where the humans wouldn't swim to, but that wasn't enough to satisfy him. His mother used to interact with humans, sing to them, touch them… Eijiro just wanted to get a little closer that's all.

He waited until early morning to swim closer to the beach, wary when he noticed the sea life thin out and the coral became lifeless. There was one human on the beach, it was hard to see from where Eijiro hid below the surface but the human had one of those boards with him.

Good, the human would stay on the surface and Eijiro could observe from below. He stayed on the seafloor and watched the human play with the waves above. Eijiro could only see the human's feet but he was mesmerized, watching the board cut through the water so fast.

Then the human fell into the water and Eijiro got his first real look at a human.

He was beautiful. Sun tanned skin covered in little freckles and his sun bleached hair was nearly white, it looked soft. Eijiro had the sudden strong urge to touch him, but the human kicked his legs and retreated back to the surface and started playing with the waves again.

Eijiro watched him for hours, staying a close for the rare moments the human fell into the water so he could catch glimpses of the perfect human. Sometimes when he fell into the water he would scream, flailing his arms around in frustration before going back to the surface.

Eijiro wondered what he was doing, why he was so angry. He wanted to see more, but he was already pushing it by staying this close for this long.

When the human started back towards the beach Eijiro peeked his head out of the water to watch him go, the water droplets on the human's back glistening in the sunlight. He turned his head to the side and Eijiro caught a glimpse of the humans burning red eyes.

The merman slipped below the surface and swam away just before the human turned completely around. 

Eijiro stayed by the beach and watched his human for weeks, in awe as the human's skin grew darker and his hair got lighter each day.

One day he heard his human's voice, angry and loud, but it was beautiful to Eijiro. He longed to make contact, to touch his soft skin and hear his voice up close…

Eijiro wondered if it would be ok to get his human alone, if he could just convince him to come beyond the reef where it was safer they could talk…

maybe Eijiro could even get his human to explore the sea with him. His angry human didn't seem to like being in the water, but maybe Eijiro could make that change.

One day his human travelled out farther than normal, riding bigger waves and Eijiro observed from below. He crashed into the water, the waves crashing down heavily on him.

Eijiro stayed hidden as he watched his human slam into the sharp coral, a cloud of blood blooming around his leg.

The merman watched with wide eyes as his human thrashed, tugging at his leg in a feeble attempt to free himself, bubbles exploding from his mouth in a muffled roar. Eijiro wanted to approach, to help, but his human's eyes were open…

so he waited until his human calmed down, his eyes drooped and he stopped flailing dangerously. 

Eijiro swam forward cautiously, reaching out a trembling hand to touch his human's face. He was warm and soft… softer than Eijiro thought he would be.

When his human didn't pull away he wrapped his arms around him, tugging gently until his leg was freed. Eijiro wanted to hold him forever, but his human's other leg was attached to a board that pulled them to the surface. His human must really hate being underwater…

Eijiro reluctantly swam to the surface, and pushed his human back up onto the board, and watched with his eyes just above water. After a couple of seconds his human sputtered and coughed, sea water flowing from his lips.

Eijiro watched curiously as his human gasped, his eyes irritated from the sea water struggled to focus on the merman. 

"Did you just?"

Eijiro squeaked, suddenly shy and disappeared into the depths.

His human didn't come back for three days and when he did show up, he had a funny looking bag over his leg. Eijiro stayed away, trying to be sneaky as he attempted to watch his human on the beach, but he quickly found that he was terrible at hiding himself.

He poked his head out of the water and their eyes met. 

"I know you're out there!" His human called out

Eijiro yelped and hid himself beyond the reef. He should probably lay low for a while…

Eijiro didn't come back until his human started playing with waves again. His foot was still wrapped in a bag, Eijiro wanted to touch it. He chewed at his lip under the water, longing to touch his human's skin again.

He didn't realize how close he drifted to his human until the blond jumped into the water. Eijiro yelped and tried to swim away but the boy grabbed his arm and tugged him back. The merman thrashed but his human held on tightly, wrapping his arms around Eijiro's waist.

Eijiro panicked and dived, going lower and lower until his human let go with a grunt. The merman turned to watch him swim back to the surface. He needed to stop getting so close.

His human came back every single day, but Eijiro stayed out of sight. He couldn't shake the desire to make his presence known, but he couldn't risk getting captured so Eijiro settled for giving his human gifts.

He'd wait for his human to start heading back to the beach and then quietly place his gift on the back of his board, before swimming as far as possible.

He started with small bits of coral and shiny rocks, but soon transitioned to large conch shells and pearl oysters. 

Eijiro reached up to place his newest gift on his human's board when a hand caught him and held tight.

"Hey" His human said softly, tugging at Eijiro's hand "come on I'm not going to hurt you" 

Eijiro rose slowly, looking at his human through his eye lashes. His hand was rough on his, holding so tight it almost hurt.

"My name is Katsuki, Bakugou Katsuki. What's yours?" 

/Katsuki/ 

Eijiro sunk until his mouth was under the water and blew bubbles, too shy to answer. 

"Thank you for saving my life"

Eijiro's brows pinched, he didn't think he had done anything that important. Bakugou's thumb was stroking circles on his hand and his heart fluttered

"I got your gifts, thank you" Baku continued, taking his hand back and pulling off the beaded necklace he was wearing "here"

The blond secured the necklace around his neck and smiled. Eijiro grabbed it, his eyes wide. His human had not only accepted his courting gifts but gave one back!

A huge smile spread across his face and he shot out of the water, pressing his mouth against Bakugou's quickly before retreating back into the depths.

His Katsuki came back almost everyday for the next two weeks, he'd play with the waves for a little bit before swimming farther out and talking to Eijiro. The merman was still being cautious, only showing himself completely when it was just the two of them.

Eijiro tried to get Katsuki to come out farther, to come into the water with him, but his mate seemed hesitant. 

"Look I want to, but I don't want to drown ok?"

Drown? Eijiro had never heard that term before. He wanted to ask when Katsuki meant but he was still too shy to talk to him. He stuck to blowing bubbles and splashing water at his mate.

Eijiro still brought him gifts and Katsuki started bringing him more gifts too, now in the form of food. His mate would sit on his board and hand him snacks while making casual onesided conversation. Katsuki would ask questions sometimes and Eijiro would answer with his actions

"Your hair changes color, huh?"

Eijiro nodded, popping fully out of the water to show his black hair and then diving back down so Katsuki could watch it change to bright red. 

"Can you talk?"

Eijiro bit his lip before slowly shaking his head yes, his mate scowled at him and he didn't like that. 

"Why won't you talk to me then?" 

Eijiro glanced at the beach, he had already stayed out too long and more people were showing up. Katsuki followed his gaze.

"You know they can't hear us way over here. Can I at least know your name?" 

Eijiro blew bubbles and squirmed, he didn't want his mate to be upset with him, but his name was probably the last thing he should tell a human.

He still didn't trust Katsuki enough, his mate wouldn't even come to him underwater. Reluctantly Eijiro grabbed his mate's hand and tugged him lightly towards the water. Katsuki jerked back, startled, then over compensated trying to stay up right and ended up in the water anyway.

The blond resurfaced quickly, latching into his board and blowing water out of his nose. 

"FUCK! You *cough* You can't just do that"

Eijiro winced and put some distance between himself and Katsuki.

His mate was glaring at him now, but Eijiro could see fear in his eyes too. Katsuki didn't trust him either. Their relationship was more fragile than he previously thought. 

"Ka-tsu-ki scared." Eijiro stated, staying about a foot away.

His voice sounded almost like a wind chime, soft and delicate and pretty. Eijiro struggled with his mate's name even though he had practiced saying it over and over. Katsuki eyebrows shot up and his eyes were wide.

Maybe Eijiro had messed up too much for him to understand so he frowned and tried again.

"Katsu-ki scared of me." Eijiro pointed at his mate and them at himself, hoping that would make what he was saying clearer.

Katsuki looked uncomfortable "I'm not… /scared/" 

A lie. Eijiro could tell by his body language, he blew bubbles but said nothing. His mate made a frustrated noise and rubbed his face.

"Look I'm not scared. I don't get scared. It's just you keep trying to get me underwater and you know the old fairy tales about mermaids! My great grandmother won't shut up about them. I thought she was just a crazy old hag but now that I know you're real…"

Katsuki sighed and tugged at his hair "you keep trying to get me to go with you so I'm… /concerned/ that you're going to drown me"

Eijiro's frown deepened. He didn't know about fairytales and Katsuki used that word again. Based on context Eijiro guessed it meant steal. 

"No, no drown" he said shaking his head, his mouth moved awkwardly to say the foreign word

"gift for Katsuki, want to show Katsuki. Too heavy to bring up"

His mate looked into the water where Eijiro was pointing then back at the merman and made a face.

"I'll drown" 

"No drown! I bring back" 

"Fish of brains, the water is at least 80 feet deep here!" 

Eijiro blinked. 

"I'll DROWN!"

Eijiro blinked again and Katsuki pinched the bridge of his nose.

"You don't know what 'drown' means"

Eijiro slowly shook his head no and Katsuki groaned. 

"I can't breathe underwater. If I go down there I'll die."

Eijiro's eyes blew wide and then he looked away, his lip trembled a little. His mate couldn't be with him.

"Hey hey don't cry!" Katsuki paddled over and touched his cheek "I just can't go that deep!"

"Kaachan!" A voice from the beach yelled "we have to go! What are you doing out there?" 

His mate cursed and pulled his hand back "fuck! I'm coming shitty Deku!"

Katsuki pulled his bracelet off and put it on Eijiro's wrist "I'll be back tomorrow, ok?" 

He watched his mate swim back to the beach and walk toward a green haired human. Eijiro's chest got right and a low growl escaped his lips.

He didn't understand this bad feeling, the merman had never been jealous before, but he knew he didn't like this other human around his mate. He didn't like that his mate went so willingly to the green haired human while he was so hesitant to hold his hand.

When Katsuki came the next day the beach was empty but Eijiro stayed a few feet below the surface. Even when his mate ventured out into open water on his board and splashed around looking for him, Eijiro stayed under and pouted.

He knew Katsuki could see him, his hair glimmered bright red under the water. After a few minutes his mate left and he sank even lower, deflated.

A loud splashing pulled Eijiro from his thoughts. A hand wrapped around his wrist and he jerked back. It was Katsuki, he was under water with weird bubble things over his eyes and a strange tube in his mouth. Eijiro couldn't care less though, his mate had come to him!

He wrapped his arms around Katsuki, and pressed their foreheads together. His mate struggled, yelling through the tube until Eijiro pulled back. The blond pointed up and started swimming, but he was so slow. Eijiro grabbed him and sped to the surface.

"Katsuki!" He exclaimed when they resurfaced, holding his mate close "Uou came to me!" 

Katsuki spit the tube out of his mouth "I told you I'd be ba- mmph!" 

Eijiro pressed their lips together again, bringing his hand around the back of his mate's head to hold him closer.

Katsuki resisted for a second before returning the kiss, slowly placing his hands on Eijiro's hips. 

"My Katsuki" he moaned, licking the seam of his lips.

"Whoa whoa ok enough" his mate turned his head to the side, but he sounded breathless "I'm in the water. We're alone. Will you tell me your name?"

Eijiro nipped at Katsuki's ear, soaking up the little gasps that escaped his lips. 

"Eijiro" he mumbled against Katsuki's pulse point, smiling when he felt his mate's heart speed up "my name is Eijiro"

As time went on the beach got more and more crowded. It was starting to get harder to see Katsuki. 

"It's the summer now" his mate tried to explain "people are visiting the island for vacation."

Eijiro blew bubbles, eyeing the busy coastline "if conditions don't change I cannot stay" 

Katsuki seemed uncomfortable, chewing on the inside of his cheek.

"Katsuki?"

"I know of a cove a little ways down the beach. It's off limits so no one will bother us." 

Eijiro's eyes lit up, and he rubbed his mate's calves under the water "alone?" 

"Yeah we'd be alone"

He beamed, but Katsuki avoided eye contact so his smile fell "Katsuki?" 

"I'll come tonight when no one can see me jump the fence" 

"Katsuki scared?" Eijiro frowned "why?" 

"'M not scared, I jusy" he looked behind him at the beach "it's nothing, I'll see you tonight ok?"

Eijiro went to the cove early to check it out. It wasn't ideal, there were a lot of sharp jagged rocks close to the shore but the areas with soft sand were too deep for Katsuki to stand. There was a sandbar that Katsuki could stand on, but at night there would be sharks.

The sharks wouldn't be dangerous, but his mate seemed timid around the creatures.

The sun started to set when Katsuki showed up, glancing over his shoulder as he approached. He didn't have his floaty board, but he had a large bag on his back.

"Katsuki!" Eijiro called, waving to his mate. 

"Eijiro give me a second! I'm going to start a fire so I can see when it gets dark" 

Fire? 

Eijiro watched curiously as Katsuki gathered drift wood and made a large pile.

When it was getting dark when the blond stopped and poured a strange liquid over the wood, he tossed a small glowing thing into the pile and the whole thing erupted in a red and orange glow. It was like a small sun on the ground. 

"Wow" Eijiro breathed, mesmerized by the sight.

Katsuki kicked his shoes off and sat right on the edge of the water. Too far away. Eijiro couldn't get that close to the beach. His mate pulled his knees up to his chest and just stared at him. 

"I wish to touch my Katsuki" the merman called, getting as close as he dared

"Eijiro" the blond sighed, rubbing the back of his neck "I'm gonna stay on the beach" 

Eijiro's face fell "why? We're alone"

"That's why I'm staying on the beach…"

Eijiro winced as if he had been slapped and shook his head "Katsuki scared of me."

Katsuki shifted "Yeah… yeah I'm scared of you"

Eijiro shrunk in on himself, hiding his face in the water. He didn't understand what he was doing wrong. His mother told him humans were easy to persuade and she had intended to harm them. Eijiro just wanted his mate to be closer, to be comfortable around him.

Now he realized why Katsuki had seemed uncomfortable this morning, the blond had been using the people on the beach as a sort of safety net in case Eijiro decided to pull him under.

What had his mother done differently? 

"Eijiro" Katsuki said "come on don't be like that, I'm still here aren't I?"

Eijiro blew bubbles under the water, thinking.

"Eijiro, hey I-" 

"Can I sing to Katsuki?" Eijiro suddenly asked, popping his head up

"Sing?"

"Yes. I want to sing for Katsuki" 

Katsuki looked incredibly apprehensive "I-I don't know, Eijiro"

"Katsuki stays on the beach" he reassured him 

The blond bit his lip and scooted farther from the water "ok…"

((Cw it gets a little angsty, but I promise a happy ending to this thread))
Eijiro smiled, his mother always sang to him when he was little and he remembered her singing to his father too. He'd sing something comforting and soft, something that would help ease his mate.

Eijiro opened his mouth and began to sing. Katsuki scowled but his shoulders relaxed. 

"Wow that's...something else" Katsuki said and Eijiro swelled with pride. 

He leaned forward on the rocks, just a little closer, and sang with more confidence.

Katsuki's scowl softened, and a smile tugged at the corners of his lips. Eijiro was happy, his mate seemed comfortable now. That's all he wanted. 

Maybe a little too comfortable… Katsuki eyelids drooped slightly, but Eijiro was too excited to notice.

He closed his eyes as he got more into the song, his tail swishing in time with the melody. Eijiro had forgotten how much he loved to sing. He opened his eyes just in time to see his mate stand up, a little wobbly, and Eijiro faltered. Was Katsuki going to leave?

"No, don't stop," the blond said, his voice sounded dreamy.  

Eijiro felt giddy, but he kept singing. He'd do anything for his mate. He wished he had tried singing to Katsuki sooner. The blond started forwards, walking towards the water, towards /him/.

Eijiro was elated, smiling so wide his cheeks hurt. He felt like he might cry.Katsuki walked slowly, staying as if he were on a boat in rough water. Eijiro was worried his mate might fall, the rocks below were sharp, but he couldn't get any closer.

He held his arms out when the blond got closer, but something seemed off. The water was already chest high on Katsuki, but he was still trying to walk to him. Eijiro stopped singing, his smile wavering. 

"Katsuki?"

The water was at his chin, but he seemed unconcerned. The blond was getting closer to a drop off but he still hadn't started swimming. Eijiro was starting to get worried, he didn't know what was happening. Katsuki took another and his head slipped under water.

"Katsuki!" Eijiro dived forwards and grabbed him, pulling his head just above the water "w-what's wrong? What happened?"

Eijiro held him above the surface as best he could, but the shallow water made it hard to balance and Katsuki was dead weight.

The blond was breathing normally even when he went under, completely unfazed by the mouthfuls of saltwater he was swallowing.

"Katsuki!" The merman screamed "what's wrong with Katsuki?! Swim please!" 

Katsuki's head lulled to the side, his eyes were barely open and he was still smiling.

"keep singing, it's beautiful" he mumbled

"Katsuki no! Swim!" Eijiro begged, a waved pushed him closer to shore and they dipped back under for a moment "Katsuki needs to swim!" 

.

Eijiro turned his mate so his back was against his chest, holding him up from beneath the surface. It was easier this way, but he couldn't keep it up forever.

Another swell pushed him into the jagged rocks and he cried out as his fins scrapped against it. 

"Katsuki!" He tried again desperately "Katsuki please wake up!"

The water was shallow enough for his mate to stand, but his legs wouldn't hold him. Eijiro gritted his teeth and started swimming towards the beach. The rocks cut his fins and tail, the sea water burning the wounds. Tears streamed down his face, but he kept going.

The closer he got the harder it was for him to keep Katsuki afloat.

"Katsuki please I'm sorry!" He sobbed, the water was shallow enough to sit in but that meant more weight pressing him down on sharp points "I just wanted to make Katsuki happy. I didn't want this."

Eijiro was stuck, blood clouding the water around them. If he had legs it would be easier, he could just carry his mate to shore. He needed legs. 

"Please" he wailed "please I need- I need to save Katsuki"

He screamed as he attempted to stand on his fins, it felt like his whole body was on fire and there was a hot knife cutting him down the middle. Eijiro put his hand down and pushed up again, crying out in agony as he felt himself split in two.

And then he was standing on wobbly half fin legs, each step was excruciating and Eijiro fell several times, the rocks slicing into his new soft flesh like glass. He didn't even have the strength to carry his mate and had to drag him, his heels scraping over the ground.

The pain didn't phase him at all, he was babbling nonsense into Eijiro's ear, lips rubbing over his tear stained cheek.

Eijiro dragged Katsuki all the way to the beach, the grains of sand felt like razor blades on his raw bleeding feet.

The hot air from the bonfire burned Eijiro's sensitive skin. He didn't stop until he was halfway up the beach and then collapsed, holding Katsuki in his lap. Eijiro pressed his face against his mate's soft, fluffy hair and cried.

"I'm sorry" he whimpered "I'm so sorry Katsuki" 

"Kaachan?" 

Eijiro stiffened and looked towards the trees where the green haired boy appeared with a flashlight. 

"I got your text… it scared me- hey! What are you doing to him!?"

The human was running towards them, and Eijiro scrambled to his feet, biting his lip hard to keep from crying out in pain. He retreated back into the sea, back over the rock until it was just deep enough to swim.

Eijiro didn't know how to get his legs to fuze back together, he didn't even know he could get legs in the first place.

Maybe it was permanent. He breathed in deeply underwater, the cool water in his lungs calmed him slightly.

Eijiro peaked out of the water to watch the green haired human help Katsuki to his feet, he looked disoriented but no longer in a trance.

"Eijiro!" He called out "Ei?" 

But Eijiro had already submerged and swam away.

His parents were right, he should have stayed away from humans.

~earlier~

Katsuki sat in his living room, bag packed with snacks, flashlights and stuff for a campfire. His great grandmother was asleep in the recliner next to him, snoring softly.

She lectured him until she passed out today, going on and on about how she could tell he was enchanted by a mermaid by his eyes.

"There's not a mermaid" Katsuki lied as he packed his backpack "I told you it's just a kid who helped me out, it's not a big deal"

"I've seen the gifts" she wheezed, pointing her crooked wrinkled finger at him "it's a mermaid. It'll eat you alive boy, lure you into the depths with its song"

Katsuki bit his lip and checked the time on his phone. He should head to the cove now if he wanted to see Eijiro while it was still light. He wanted to watch the sunset together, but his old hag had gotten into his head.

"He isn't going to hurt me" Katsuki mumbled to himself "if he was gonna hurt me he would've done it when I was already drowning" 

He stood up and quietly snuck outside. He rode his bike to the fence surrounding the cove.

It was a private section of the beach on the Todoroki estate, but there weren't any cameras or anyone on the beach. He leaned his bike against a tree before grabbing the fence. Katsuki was nervous. He groaned and ran his fingers through his hair before pulling out his phone.

"At the cove. If I'm not home by 10 come get me. If you don't see me on the beach I might be in the water" 

Katsuki took a deep breath before sending the text to Deku. The shitty nerd stayed at his house every summer while their parents went on extended vacation.

Katsuki hated Deku, but the nerd would stay up to make sure he got home.The blond climbed the fence and made his way to the cove. He didn't see Eijiro immediately and was almost relieved. 

"Katsuki!"

His heart skipped a beat and he spotted Eijiro waving at him from the water. The way Eijiro said his name made Katsuki feel warm and fuzzy. The mermaid was far away and it made Katsuki uneasy. He kinda wished Eijiro had been on the beach.

"Eijiro give me a second! I'm going to start a fire so I can see when it gets dark" 

It was a lie, he had flashlights so he could see. The old hag had told him mermaids were scared of fire, it seemed safer to have one.

Katsuki could feel Eijiro's eyes on him while he gathered firewood, he felt like a small animal being stalked by a predator.

The fire roared to life and he checked back in Eijiro. The mermaid didn't look scared at all. Katsuki huffed and sat near the edge of the water, the waves touched his toes but he recoiled from it.

"I wish to touch my Katsuki" 

His heart raced. He liked when Eijiro said "my Katsuki" but he knew how dangerous it was too. For whatever reason this mermaid had claimed him, saved his life, courted him and taken his first kiss

And fuck Eijiro was addictive, every touch and kiss made Katsuki want more. He tried so hard to stay away but he ended up heading to the beach without thinking about it. 

"Eijiro, I'm gonna stay on the beach" it was hard to say, he didn't want to stay on the beach.

Eijiro's frown made Katsuki take his words back.

"why? We're alone"

"That's why I'm staying on the beach…" 

His great grandmother warned him that this is what mermaids do, they seduce you and then once they get you alone they drown you and eat you.

Eijiro didn't seem like he wanted to waa at Katsuki… at least not in the literal sense. But this was the first time they were completely alone together and Eijiro was always so adamant that Katsuki get in the water. 

"Katsuki scared of me."

Fuck, Eijiro looked like a kicked puppy. His big red eyes shiny with tears and his bottom lip trembled. He made Katsuki feel so guilty for not trusting him.

"Yeah… Yeah I'm scared of you" Katsuki admitted, his heart breaking when he saw the effect his words had "Eijiro, come on don't be like that, I'm still here aren't I?"

He wanted to fix this, he hated seeing Eijiro upset even though it was kind of cute how he blew bubbles when he pouted. The mermaid acted like a baby sometimes, how could someone so innocent be dangerous?  

"Eijiro, hey I-" Katsuki began to apologize but was cut off.

"Can I sing to Katsuki?" 

His blood ran cold "sing?"

"Yes. I want to sing for Katsuki." 

/Red flag, red flag, red flag/. Everyone knew that's how mermaids lured people to their deaths.

"I- I don't know Eijiro" his voice cracked, he should have said no, but he couldn't.

"Katsuki stays on the beach" 

The blond tried to swallow the lump in his throat. Eijiro didn't want to hurt him… he wouldn't hurt him… Katsuki backed up away from the edge and hesitantly agreed.

He didn't know what he had expected Eijiro to sound like, but it was nothing like what the mermaid actually sounded like. His voice was amazing, it didn't sound human (well he wasn't human but still). It sounded like Eijiro was playing a musical instrument,

somehow hitting multiple pitches at the same time. Katsuki couldn't stop staring.

He felt his body relax on its own and he frowned. Katsuki felt like his brain had been wrapped in a weighted blanket. It was hard to think. 

"Wow that's… something else"

Katsuki felt like he was talking in slow motion. The world fell away and suddenly there was only him and Eijiro. The blond felt fuzzy and warm, like he had taken three shots of vodka but without the burn. He wanted more of that feeling, he wanted to hear Eijiro's singing more.

He was too far away. Katsuki stood up, his body wobbling like he was drunk. The song got quieter and a Katsuki felt a twinge of panic. Eijiro couldn't stop now! He needed more.

"No, don't stop"

Wait, had Katsuki said that or did he just think it? He couldn't tell… was he in the water now? Eijiro's smiling face was getting closer. His mind vaguely registered that there was cold water up to his chest now. 

"Katsuki?"

Oh yeah that's his name. Eijiro was calling him. Katsuki wanted to respond but he felt so heavy. What was he doing? He couldn't remember. He felt like he was falling and then everything went black.

"Kaachan! Kaachan what happened?! What did you take? What did he give you?"  

Katsuki groaned, his mind still felt fuzzy and his limbs were heavy. Deku helped him to he's feet and he bared his teeth at the green haired nerd. Why was /Deku/ here? And why…

"Why am I wet?" Katsuki grumbled, wiping the saltwater off his face. 

"I don't know! Kaachan who was that guy? Were you doing drugs? What happened?" 

"Guy…?" Katsuki's eyes shot open and he looked out at the dark water "Eijiro!" He yelled, scanning the surface "Ei?"

Katsuki couldn't see the mermaid anywhere. 

"Kaachan who was that guy?" Deku demanded 

The blond took a deep breath "Deku you're not gonna believe me…" 

Katsuki was fuming as he sat in the small walk-in clinic on the island. Fuyumi had just run several different drug tests on him. Deku was sitting in the corner talking to his boyfriend, Shoto. Shitty nerd had connections with the most powerful family of the damn island.

"There doesn't appear to be any traces of substances in his blood" Fuyumi said with a soft smile, stitching up some of the deeper cuts in his heels. "There's evidence of raised endorphins, but he could've had the same reaction from going on a long jog"

"Told you assholes" Katsuki grumbled, winced as he got off the examination table. 

Deku and Shoto helped the blond hobble out of the clinic. 

"I need to borrow a boat, half-in-half" Katsuki said, matter of factly "and scuba gear"

"Kaachan it's midnight! You can't seriously want to go out to find this 'merman'" Deku said alarmed "I'm taking you home" 

"You fucking saw him!" 

"I saw a scared kid touching you while you were passed out. I think we should've called the police"

"Didn't you say he escaped into the sea, Izuku?" Shoto asked calmly, putting Katsuki in the backseat of his car "I'm not saying I believe Bakugou, but that is strange. Plus the way Bakugou's heels are cut up that boy must have dragged him out of the water"

"Yeah shitty Deku" Katsuki hissed, crossing his arms and sinking into the seat "you fucking scared Eijiro off, I have to find him /now/" 

They made him wait until the morning to search for Eijiro. Katsuki started at his usual spots but didn't see any sign of him.

"Bakugou…" Shoto said setting up a small motor boat "why do you want to find this mermaid so badly? Don't mermaids kill people?" 

"Shut the fuck up"

It took three days for Eijiro's legs to turn back into a tail and it hurt almost as badly as it did when it split apart. He stayed away from the beach, but he couldn't bring himself to leave the island completely.

He wanted to be near his mate even if he couldn't see him. There was no way Katsuki would want to be around him now, he had good reason to be scared of him. Eijiro felt like a monster.

He stayed underwater for almost a week, collecting shiny objects and shells to build a nest. Eijiro found an underwater cave on the other side of the island that had a nice big air pocket inside.

Good for his mate. Eijiro gnawed at his lip, swimming outside to check the stars. This was all terrible timing.

Eijiro went out the next day to look for more shells, picking up ones he thought Katsuki would like without thinking. He was craving one of the snacks his mate brought him one time… potato chips…

Eijiro was inspecting a large conch shell when someone touched his shoulder. He wiped around and found his mate wearing the funny eye bubbles (Katsuki called them goggles) and tube in his mouth that was attached to a metal tank in his back.

Eijiro squeaked in surprise and shoved the shell into Katsuki's hands before turning to swim away. 

Katsuki let out a muffled shout and grabbed Eijiro's tail. He looked back with wide eyes and his mate pointed to the surface before letting him go and swimming up.

Confused Eijiro followed Katsuki to a small boat and watched him climb up onto it.There were two other humans there, the green haired one (which made Eijiro growl quietly) and a strange one with half red, half white hair.

The green haired one gawked at him, while the other helped Katsuki remove his gear.

"Katsuki…" Eijiro said hesitantly "these hoomans…"

"They're my…" Katsuki glared at them for a second before rolling his eyes "They're my friends, Deku and Shoto. It's ok, Ei."

He shook his head and blew bubbles to avoid growling that the green haired one. His mate extended his hand over the side of the boat. 

"Can you come here?" 

Eijiro shied away "Katsuki, I cannot"

Katsuki frowned but kept his hand outstretched

"My Eijiro" he said softly "come on"

Eijiro's face flushed and he toyed with the necklace Katsuki gave him.

"Katsuki's Eijiro" 

"Yes"

Eijiro beamed and took Katsuki's hand, allowing himself to be pulled aboard. 

"Fuck you're heavy" the blond groaned, needed the other humans' help to bring Eijiro on deck.

They sat the merman down on a chair, Katsuki fished around in a cooler and pulled out a colorful, layered square thing on a plate and handed it to Eijiro.

"What is this, Katsuki?" 

"It's called cake, Eijiro"

"Oh" he stuck his index finger in the squishy colorful cake and put it in his mouth, his eyes grew to the size of dinner plates and he shoved the rest of the slice in to his mouth "sweet"

"I wanted to talk to you alone" Katsuki said "but I wanted to prove these two idiots that I'm not crazy" 

"What was I supposed to think when you said you were dating a mermaid?" Deku grumbles.

"Shut the /fuck/ up, Deku!" Katsuki hissed "I didn't say dating. He courted me and I… I reciprocated" 

"My Katsuki" Eijiro nodded, his mouth full if cake as he eyes Deku

"So you're engaged to a mermaid?" Shoto asked while pulling up the anchor.

Katsuki's face burned and he cursed under his breath, taking a seat next to Eijiro.

"Engaged?" Eijiro questioned looking at Katsuki and then at Shoto. 

"You know, husband's? Partners?" Deku offered 

"We're not!"

"Mates" Shoto shrugged

"Mates!" Eijiro perked up "Yes! Katsuki is my mate!"

He leaned over and nuzzled Katsuki's neck, nipping lightly at his skin. The blond bit his tongue hard to suppress a moan and pushed him back. 

"That's enough" 

"No it is not" Eijiro murmured, sending shivers up Katsuki's spine.

The merman's pupils were blown wide, his eyes half lidded with lust, and he was panting. He leaned back into Katsuki's space again, lips ghosting over his jaw.  Deku was blushing a deep red and avoiding eye contact while Shoto looked on disapprovingly. 

"Not on my boat please"

"Nest" Eijiro whispered into Katsuki's ear "made nest for Katsuki" 

"Kaachan!" Deku sputtered and Eijiro hissed at him. 

"Bakugou, this creature clearly wants to mate with you…" Shoto said "I think it would be a good idea to put him back and go."

"Nest. Mate come" Eijiro's voice dropped, low and raspy. Katsuki shuddered. 

"I-I can't" he breathed, scooting away "Eijiro no"

The merman whined "then I must go"

Katsuki was visibly torn as he watched Eijiro flip over the side of the boat and splash back into the water. 

"Kaachan…" eyeing the blond as he stared into the water "think rationally about this" 

"Or don't." Shoto shrugged, and Deku shot him a glare. "Just make up your mind."

The blond could see Eijiro was still there, his red hair shining under the surface. He took a deep breath before deciding to push logic aside. When would he ever again get an opportunity like this? Katsuki stood up and climbed up on the edge, the small boat tipped.

"Kaachan!!!" Deku yelled, grabbing his wrist "Kaachan you can't! This is exactly what they do in the stories great grandma told us! Please think about this!" 

Katsuki jerked his hand back. 

"stay back, Deku" he spat and then dove into the water.

The blond opened his eyes, the salt water stung as he swam deeper. He could hear Eijiro's voice below the surface as the merman sang to himself. Katsuki's brain immediately felt fuzzy again. 

Shit. 

"Eijiro!" He yelled, his voice distorted in the water.

Eijiro turned and smiled at his mate, a different, darker smile than usual and his eyes had a dangerous gleam to them. It threw Katsuki off, a primal instinct told him to run, that this was a mistake.

He tried to shake the scared feeling away, but it didn't help that Eijiro swam up to him so quickly. Their chests slammed together and the merman shoved his tongue into Katsuki's mouth, he tasted like salt water and cake.

"My Katsuki, my mate" Eijiro cooed, his voice was even more enticing underwater "Katsuki trust me." 

The blond wasn't sure if it was a question or a statement, but he found himself nodding anyway.

His lungs burned, he needed to breathe, but with the way Eijiro was looking at him he almost didn't care.

Eijiro wrapped his arms around Katsuki and swam off, going faster than the blond could even process. They stopped once on the way to the nest, Katsuki was thrust above the surface just long enough to gasp before Eijiro pulled him back under and sped the rest of the way.

Eijiro sat Katsuki on a ledge in an air pocket That was deep in an underwater cave. He never even knew the place existed before now. He and Eijiro were completely and utterly alone. Katsuki squirmed, suddenly hit with second thought as his grandmother's words burned in his mind.

"Eijiro…" Katsuki breathed, his words echoing ominously. 

Eijiro, however, was already working off Katsuki's swim shorts "Katsuki has always hidden Katsuki's full self" his fingernails extended into claws and tore through the material.

Oh fuck. Katsuki reclined on his hands, throwing his head back as Eijiro licked and nipped the inside of his thighs. Eijiro grabbed his hips, pulling him closer, Katsuki almost slid off the ledge.

The blond ended up wrapping his legs over the merman's shoulders, his lewd moans bouncing off the cave walls and Eijiro's mouth got closer and closer to his throbbing length.

"Oh fuck please" Katsuki mewled, his hips jerked towards Eijiro's face. 

The merman's tongue darted out, licking gingerly over Baku's slit and collecting the bead if pre at the tip. 

"Sweet" Eijiro purred and Katsuki writhed, desperate for more contact. 

"Eijiro, /please/"