a siren's song is all about their victim's greatest desires.

siren!chuuya's song is somehow unable to affect human!dazai?? so they end up having a bet as to whether chuuya can eventually sing smth that will work on dazai.

is it bec dazai somehow has nullifying powers? is dazai really just an outlier? are his desires so out of this world that not even a siren can understand them? or is it because dazai doesn't want anything that badly? or is he just that emotionally constipated??

chuuya is Affronted that someone of his caliber can't read dazai's deepest desires. so he tries to his best to understand.

....just so he can win that bet, of course.

chuuya refuses to sing a shinjyu song. first, out of Principle(TM). second, he doesn't think dazai Really desires it anyway.

he tries a song about crabs and seafood? about bandages? about being a bastard who can predict everything??

anyway, dazai tells him that it's ok if he wants to admit defeat, he didn't really have much hope on sirens.

turns out, dazai had tried to meet sirens so he could find a direction in his life. hoping that a song about his deepest desire would make him understand what it is.

chuuya is Even More Affronted, and resolves to discover it, No. Matter. What.

...just so he can win the bet, maybe...? also bec he wants to see dazai's expression once he proves him wrong...?

they end up spending a lot of time together --- chuuya, bec he wants to discover it; dazai, bec it's fun to lord over his ~superiority~ when it comes to knowledge about the world.

chuuya would be like, "i bet you can't explain away just why the sea is so beautiful" & dazai responds with a physics lecture about color reflection and such.

eventually, they spend so much time together that the other sirens are getting suspicious that chuuya spends so much time near the land. they try to warn chuuya that humans are a greedy bunch -- that's why siren songs are so effective.

chuuya doesn't tell them about dazai--that's his own matter, after all. he tells them that he's just a bit interested in the outside world, hoping that it will make them back off. (not knowing that it just increases their suspicions)

anw, one day chuuya is a bit stressed from sheep (but, like. under the sea version) hounding him about his trips to the shore, and dazai is more annoying than usual, so chuuya irritably sings about dazai drowning in the sea to shut him up.

at this point, he's pretty much already forgotten about their bet, since he's kind of figured that dazai must be immune to siren songs to begin with.

so, he's definitely not expecting his song to actually somewhat work --- dazai does start walking deeper into the ocean.

chuuya panics -- he truly did not expect that dazai's desires would actually respond to that song??? he didn't think that dazai still had any true desires towards death??? haven't they been having lots of fun?? is he not enough to make dazai want to live longer??

but first things first -- he needs to stop dazai from drowning. he tries to pull dazai back to the shore, but just as he's trying to lug him back ("how are you so slippery, are you a damn mackerel or something!!!")

...his comrades from under the sea show themselves.

they've apparently followed chuuya up this time, because they want to know just what is occupying chuuya's time so much.

and now, they're convinced more than ever, that chuuya has betrayed the secrets of sirens to humans. that he's siding with humans instead of fellow sirens.

chuuya doesn't have time for that??? he's got bigger (also kinda literally) things to worry about!!!

he asks them (yells at them, really) for a way to undo the effects of a siren's song.

it's apparently The Wrong Thing to ask, because is chuuya rejecting his nature as a siren??

why would he want to undo something that sirens take pride on?? why is he having second thoughts about using their main weapon?? is he really a traitor to their kind after all??

but while they're arguing, dazai uses the opportunity to get back to following the stipulations of the siren song... and tries to go deeper into the sea.

chuuya catches him, but they're already too deep in the waters.

chuuya hates the fact that dazai wears so many layers that weigh him down ("i knew i fucking hated these damn bandages at first sight!!") so while trying to drag dazai back up, he starts stripping off dazai's clothes.

so he can be lighter and stop him from sinking so fast, okay.

once dazai's p.much down to just some bandages and his underwear, they're still p.deep into the sea --- but dazai wakes up from the trance of the siren song.

and because he wakes up in the middle of the sea, he... starts choking and flailing.

in a last ditch effort, chuuya tries to calm dazai down and give him air ..................................................................................................................................................................................by punching him in the mouth.

...using his mouth.

it's not a kiss, okay?? he's just transferring air!!! and he just wants to shock dazai into stillness so he'd stop flailing and fussing and choking and just. stay. still. so he can bring him back ashore.

dazai does end up very shocked, and also very still and quiet. and so chuuya uses that chance to bring him back up.

and also a chance for him to rant angrily about dazai's shitty desires.

"how could you want to drown in the sea!!! it's full of seaweed and it's really dull and dark underneath!! you know so many things and instead of using your genius to think of ways to make yourself happy, you're gonna just give up and die?! and i actually thought that you liked--

--spending time with me! was i just filler for your time?! when you said that you liked crab, when you said that you liked your friends, when you said that you liked your protege, when you gave me that totally unnecessary physics lecture, were you thinking of just leaving anyway!

...chuuya stops himself there.

a moment, before dazai says, "i never thought of leaving you."

"g-good!" and then, "who said about leaving me specifically!!! i don't care about that at all!"

"so you just cared about me in general?"

"what i care to know is why would you still have such a desire to drown when--"

"...it's not a desire to drown, per se, chuuya."

chuuya waits for an explanation, but dazai goes back to being quiet. he does seem a bit embarrassed, but chuuya's more concerned about getting the truth out so he tries to shake the words out of dazai.

dazai ends up sighing, "the part that resonated was about being one with the sea."

chuuya..............................does not get it.

since dazai conveniently says this just as they're coming up for air (and therefore, near the waiting sheep who've been waiting for chuuya to return there to finish their argument)...

...sheep interprets this as a filthy human wanting to know the secrets of the sea/sirens.

"chuuya! this filthy human is finally showing his true colors! his true desire has to do with becoming one with the sea?! isn't he just wanting to learn the secrets to our power?! isn't he just wanting to become a siren in the end?!"

and that's... what makes chuuya get it.

he turns to dazai and asks, incredulously: "so it worked because you wanted to follow me down to the depths??" and then, "your deepest desire is to stalk me that badly??"