You know that hc that in a/b/o- betas are capable of switching their second gender if needed? What if only galra betas can do that?? Also that the second gender in the galra species is enhanced in order for them to mate with multi-species and grow their numbers.
Now, beta!Keith.

#prekerb #friendsheith (eventual #sheith, mentions of a//dashi)
Shiro has no fucking idea why the smell of Keith-- a simple beta, is so fucking charming and soothing to him, even more so than the scent of his omega boyfriend, Adam.

Keith thinks he's a normal beta and doesn't notice his body is slowly changing and adjusting into certain omega traits bc he is reacting to the distress of alpha! Shiro, his only friend whom Keith cares for more than anyone.

Shiro is always stressed- there's his position as the golden boy of the Garrison and the reputation he must mantain due to it, there's Kerberos, there's Adam being overprotective which leads to them fighting more and more often.
Somehow, only Keith's company eases his nerves.

Keith is slowly allowing physical contact, and Shiro is a little ashamed of how obsessed he instantly becomes with the smell of Keith after their very first hug.
One accidental sniff to Keith's hair and his nostrils are invaded by the soothing smell of mint and lemon verbena.

It calms him immediately, he forgets whatever he was pissed about- another disdainful comment from Admiral Sanda, and he clings to the hug until Keith awkwardly clears his throat because Shiro's pleased, possessive petrichor scent is all over the place and it's enough to

overwhelm even a clueless beta like Keith.
That.
That never happened to Shiro before.
Not even for his boyfriend.

Shiro tries not to be obvious about it, he doesn't want to scare Keith off or weird him out.
But he scents Keith.
It's addictive how quickly tension leaves Shiro's body even after one sniff. And a little scary, too, how sometimes it even

manages to ease the pain on his right arm.
He tells no one about it, especially Keith and even more especially Adam.
It's too weird, too shameful.
Rides through the desert help a lot because if he lets Keith take the lead, the wind pushes Keith's smell right on his face and he

can enjoy scenting Keith guilt-free.
Sparring together is harder. Especially when Keith is pinned under him, thrashing, and Shiro accidentally gets a good

inhale of Keith's hair. It distracts and relaxes him almost instantly so Keith manages roll him over and get the upper hand.
Keith is not happy with Shiro going easy on him, but Shiro can't even defend himself, he's too content and pleased with his omega--
Wait.
What.

*
The breakup doesn't actually come as surprise. It was doomed as soon as Shiro took the Kerberos mission.
They did try to fix it, until it was clear than the only thing that would satisfy Adam is Shiro abandoning his dreams.
It stings but doesn't shock him, but

Shiro does feel guilty about how his body began to recoil from Adam's scent after the last argument.
He doesn't mean to offend Adam, but he can't take the smell anymore and after weeks of coming home late to avoid him and sleeping on the couch, Adam takes the rejection for what

it is, and offers Shiro to break up.
Shiro feels really bad, and even cries because he just lost a promising bondmate, and Keith comes to help scrub the place clean because the smell is too haunting and it induces so much guilt in Shiro it makes him nauseous.

His body feels better by the time they're done, like it always does after he gets to scent Keith.
But the guilt stays, gets stronger because now his place smells of Keith, and Shiro really, really likes it.
And it's just weird.

Keith isn't an omega. Why is his smell so strong?
Why is it so fucking good?

*
It's summer break, and Keith is given the choice to return to the Home, stay in the Garrison, or spend it in Shiro and Adam's apartment like he's been the past few years.
Well, just Shiro's apartment, now.

Now, Shiro has a problem.
He always schedules his yearly rut on summer vacation- on the one week before Keith comes to stay, because he still has extracurricular SIM sessions to make up for any discipline issues that may have affected his grade through the year.

This is going to suck because:

1) He doesn't have an omega to share a rut with for the first time in years.
2) On the latest year Keith grew a little more composed- almost submissive, with just small sprouts of rebellion and

snark, but nothing bad enough to affect his grade. He's coming to spend the full summer here.
And Shiro is about to go off his suppressants.

No one sees a problem with Keith staying over because Keith is a beta.
But for some reason, Shiro feels awfully stressed about it. One part of him is thrilled becauseKeith smells nice, Keith will take care of him, Keith Keith Keith, Keith.
The other part is scared shitless of

doing something he shouldn't to Keith because this already feels like a bad rut and Shiro is full of violent energy, possessiveness, and pent up sexual tension that only an omega can tolerate.
He feels all will be directed to Keith. Neither makes sense because he isn't an omega.

(Warning for nsfw-ish ahead because Shiro is going through his rut, which means a little horny, a little feral and not very coherent)

The first day of his rut is really, really bad. He doesn't remember it ever being so bad.
As soon as Keith enters the room and his scent hits Shiro's nostrils, he finds himself salivating.
Keith is greeted with the sight of his best friend standing frozen

in the middle of the living room, shirtless, pupils blown wide and drool dripping down his chin to the floor.
The concious bits of Shiro are thankful for how Keith doesn't judge him as he approaches, cleans Shiro up and takes him to the bedroom.
His smell gets stronger, though.

It's addictive. Shiro realizes, late, that his knot brain is taking over and is pulling him to get closer to Keith as they walk to the bedroom.
Get him into your nest. Smell his hair, latch your mouth to any skin Keith allows to touch.
Touch touch mouth mouth touch smell mouth-

"I want to put your hair in my mouth"

Keith blinks at him then laughs, replies with a mocking tone, something about never letting the golden boy live this one down, and Shiro feels like a failure because Keith doesn't get it.
He doesn't get it and Shiro needs him.
He needs his omega.

It gets even worse because since Keith isn't an omega, he has no idea of what omegas should do. So when he sees Shiro's makeshift nest on the bed he doesn't immediately compliments it... Shiro freaks out. He's pacing around the bed, whining loudly, so hurt by his omega's

rejection that he tears up, which in turns freaks Keith out.
"SHIT- buddy what's going on? Shiro?"
But Shiro can't voice himself properly right now, because he's overwhelmed and stressed and at some point between the small walk from the living room to his bed he's gotten hard

And it hurts and-- he can't explain any of it.
He can only sob "why?" and then "please"; the sight of seeing the one and only Takashi Shirogane crying for no reason shocks Keith enough to make him take a few steps back, his knees hitting the bed, and it feels in slow motion when

Shiro watches Keith fall right into his nest.

If Shiro were more himself, the sight would've been funny.
Keith is in his bed, stiff as hell, with his hands raised palms open, as if afraid Shiro would attack him for getting into his nest.
But that's exact where Shiro wanted him, and it's the only thing Shiro registers.

He's still sniffling and hiccuping when he nervously crawls into the nest and hovers above Keith, waiting for permission on something.
Shiro himself doesn't know what.
Keith blinks a few times and slowly begins to relax, and maybe is because he can see Shiro still visibly upset

but his scent /blooms/. It gets stronger, and heavy and it weights on Shiro's entire body just as it also makes his dick twitch.
Keith licks his lips and Shiro watches. He can feel the fog in his mind lifting, too, super slowly, and his tongue feels like cotton.
"Keith."

Keith perks up. "Hey."
Shiro knows he wants to say something, but he doesn't know how to yet. His eyes flicker to Keith's face and the mess of sheets and pillows on the bed around them a few times before Keith finally picks up on it.
"Your nest?"
Shiro grunts, nods.

"You like?"
Keith searches around Shiro's face for something, then smiles. "Yeah, bud. Of course."
Shiro chirps, pleased, and lowers enough to give Keith an affectionate headbutt on the chest. The room smells so nice, and Shiro grows slightly more lucid.

"Keith."
"Mmmm?"
"You smell nice."
Keith blinks. "Okay."
"I'm going to sleep on top of you."
Keith blinks again. "Oh- okay."

Shiro's body feels so heavy, and Shiro doesn't have enough shame to mind his hardon when his arms give out and he flops his entire weight on top of his best friend, who lets out a loud "oof" and a wheeze.

And now that Shiro finally got to lie down with his omega, he feels concious enough to remember Keith is not his omega. Or an omega at all.
But he smells so good, it's intoxicating, and Shiro is lucid enough to realize he can't fucking move.
He wants to apologise for the

hard dick pressed to Keith's hip, but Keith isn't saying anything about it. Shiro is gonna find a way to make up for it.
Keith does bring one hand to scratch Shiro's undercut, and it feels so fucking good.
Shiro feels so tired, so exhausted.

He makes one last effort to wiggle up until he can hide his face on Keith's neck to scent all he wants, and falls asleep.

They manage something for the next few days, and it becomes their system for when the doctors need to trigger another rut on Shiro a few months later, since he won't be able to have one during the Kerberos mission.
Keith's clothes go to the nest. Keith has to wear

Shiro's clothes, which will also go to the nest later.
Keith gets banished from the bedroom whenever Shiro needs to lock himself in and pop a knot, and the worst he has to deal with is the howling and growling from the door.

Shiro eventually comes out because he wants Keith to feed him, and help him bathe, or simply to take naps on Keith's lap.
He is a gentleman, and has enough mind to clean up after himself and put clean sheets and new clothes before leading Keith back to his nest at night to

sleep, and so Keith and impregnate the nest with his scent again.
They don't exactly talk about why does Shiro need Keith to scent him and his nest so much.
But when Shiro recovers, he does try to talk about what it'll mean for them, when he comes back.

Keith cuts the conversation short, saying he still offers to share Shiro's ruts as many times as Shiro needs- or until he finds an omega partner.
It makes Shiro thankful for Keith's loyalty, but it worries him that when he hears "omega partner" Shiro can only imagine Keith.

**
Shiro goes through three painful, tortuous ruts during his year in captivity. The galra know what he is, and purposefully keep him without suppressants to stress him out and get him paranoid and more violent during his battles in the arena.
Each one was so awful Shiro

almost wanted to give up and die. Memories of his omega, Keith, would fill him up during those hard times. And how Keith promised to be there for him when he comes back, and take care of him like he's done in the past.
Sometimes it soothes and gives him hope, makes him

(NSFW warnings from here on)

Want to survive to see him again. Sometimes it just haunts him, because what is he doing, thinking he can get out of here? He'll never see Keith again.
He wails and closes a fist around his knot and calls for his omega to come take care of him.

It's traumatic enough for Shiro's ruts to stop almost entirely after the third one. No suppressants, no aid, nothing.
He just doesn't have them anymore.
The witch went as far as to try and trigger one, but only ended up getting Shiro sick, and useless for the arena for the next

few days.
It's during the treatment of that mistake that they find out his illness, take his arm, and cure him.

*
He finally, though accidentally, triggers a pseudo-rut when he gets to Earth and wakes up in the shack with Keith by his side.
Keith. His omega.
It crashes down on him and he's not exactly gentle when he shakes Keith awake and pulls him into the bed with him.

He buries his nose on Keith's hair and is clumsy while trying to breathe and inhale his omega scent while crying loudly, gasping sobs.
If Keith is upset about it, he doesn't say it. He does tense up when Shiro pulls the collar of his shirt and bites down on his shoulder until

Shiro can taste copper. Keith may scramble a little, grab the sheets tight, but takes the bite without making any sound that could stress Shiro any further.
It may be nostalgia talking, but Keith smells so fucking good, and Shiro is so hard already.

He's about to ask Keith for whatever Keith would give him; permission, consent, attention.
But it's like even with the time they spent apart, Keith grows immediately in tune with his alpha's needs. Shiro barely has the seconds to feel proud of his omega before Keith

climbs his lap and grinds his ass down on Shiro's hard cock. He grabs Shiro's face and kisses him all over, doesn't stop moving until Shiro comes, hard, whining around the wound he's leaving on Keith's shoulder.
He helps Shiro bathe later, and gives him new clothes.

They're too big for Keith, but they smell perfect like him, and Shiro smiles while putting them on.
The next morning, they don't have time to talk about what happened.
They don't address it, and it stays like that.

***
Shiro's hormonal cycle is a fucking mess. His ruts come back out of nowhere and they're awfully irregular. He can not have them for a couple months, and have two ruts in a row in the next. It's exhausting.
Not even all of Coran's attempts at suppressants can fix it.

They simply don't do the suppress bit that's so important. It's like his body doesn't give a fuck, and forces him into a rut anyway. The good thing is that they're short- they last a maximum of two days, and a minimum of twelve hours, give or take, if he gets to knot.

It was never negotiated between them, but Keith actually begins... taking care of that, too. He just does.
Logically, Shiro knows Keith is his best friend and a beta. He doesn't chirp or thrill at Shiro, even if it's been pretty clear by now that they're mates (Shiro thinks.)

He barely reacts to any of Shiro's attempts to court him as a mate and tends to tense up whenever Shiro tries to take care of him (as he thinks he should, because he's Keith's alpha mate). Keith doesn't nest, he doesn't have heats.

But as soon as Shiro needs him- especially when a rut hits, Keith is there. Shiro's never seen a more diligent omega.
Keith smells amazing, he spoils and feeds and bathes Shiro, meets Shiro's every need without him having to voice it first. He handles Shiro's ruts masterfully.

This is where it gets confusing for Shiro.

There's the primal side of him, who keeps internally referring to Keith as his omega mate, and it gets harder and harder to hush that part of him when Keith lets him fuck his thighs and even knot on his mouth. He doesn't choke, doesn't complain if his jaw hurts

("It doesn't, man, why do you keep asking?"), and lets Shiro scent and bite and kiss him while his small calloused hands squeeze around Shiro's knot and milk his seed. He doesn't mind if Shiro leaves marks on his body or if everyone notices, in fact he is almost proud to show

to show them off, like any mated omega would.
On a good day, his brain tells Shiro that Keith is his best friend and a beta he's very much pining for.
But every inch of his body sees Keith as his perfect omega, already bonded to him and only him.

He is almost sure that if he asks, Keith would present to him.
But also, every time he chuffs at Keith and Keith doesn't purr back just... well. It's a rejection, even if Keith doesn't notice. And it stings, and it brings doubt within him.

***
Keith is quiet after Kolivan returns to the base. Allura is very obviously ignoring him, and Shiro is anxious and worried for his best friend.
There's an awkward silence as the entire team tries to wrap their heads about this new information, about galra that want to

take down Zarkon, about the new concept of /trusting/ galra and realizing the blade of marmora is their current only ally.
The fact that Keith is galra.

Shiro doesn't even stop to think about it, doesn't even hesitate on his own feelings. He's as sure of Keith's loyalty to Voltron and as he's sure of Keith's fondness for Shiro.
Keith is Keith.
Galra may have hurt Shiro, but he knows by now that the species don't actually dictate

what makes you a just person, galra had hurt him and galra had saved him.
The man who takes care of Shiro with so much devotion, takes an role he shouldn't even do for Shiro's ruts- for Shiro's health, is half galra.
Nothing will separate Shiro from his omega.

He knows Keith will need reassurance, and after a brief final meeting with Allura and Coran- in which he actually went on a polite glare contest with the princess, he goes to Keith's room.
He makes sure his scent is strong and soothing.
His fist barely land on the door before

Keith speaks.
"Come in, Shiro."
Shiro tries to squash down the pride of having his omega recognize him from scent alone. And instead he is more worried that he can barely smell Keith as he enters.