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Since the day he was born, Katsuki’s baby brother had belonged to him.
Katsuki remembers being able to hold Izuku for the first time, remembers how small he was in his arms-- even if Katsuki had been a little thing himself back then at just four years old. He remembers their mom looking him in the eyes and saying, “Katsuki, he’s smaller than you so you have to look out for him, okay? That’s what big brothers do.”
And at that moment, Izuku became his. That was his Deku, and nobody else’s.
.
They grew up as close as they could be. They always played together, and more often than not, Katsuki found himself going to sleep with an armful of baby brother. Their parents thought it was cute. Izuku loved his Katsunii, and his Katsunii loved him right back.
Izuku thought that Katsuki hung the sun and the moon. Katsuki got his quirk shortly after Izuku was born. It was a powerful one, and as soon as he was old enough to, Katsuki loved to give his little brother demonstrations. He loved how Deku’s eyes would light up and he would shout, “Katsunii, sugoi!” With cheeks flushed from excitement and face bright with wonder. Katsuki was going to use his quirk to protect people, most of all, his Deku.
It didn’t even occur to their parents or either of them that their closeness might not be a sustainable thing until Katsuki turned thirteen and presented as an alpha.
On the night where his first-ever rut began, Katsuki woke to his teeth hovering just above his brother’s shoulder, poised to mark and claim. His startled noise had woken up Deku, who had started to cry like the baby he was at his brother’s new, scary fangs. The crying had woken up his parents and that had caused things in the Bakugou household to change. Dramatically.
“He’s not anything yet. I don’t see what the big deal is!”
Inko looked at Katsuki with those big green eyes, the same ones that Deku had. “Sweetheart,” she started, her hand on her son’s knee. They must have thought Katsuki would get mad. Inko only ever talked to him if they knew Mitsuki would lose her mind at him. “It’s just inappropriate for you two to be sleeping together still.”
“That’s not fair!”
“Katsuki,” she said, her voice so soft like she wasn’t about to break her son’s heart. “I know that it can be very difficult to control your urges now that you’ve presented, but what if he ends up being an omega?” She took both of his hands into her own. “You two have just reached that age where it’s not okay anymore.”
Katsuki knew he was crying, frustrated tears spilling down his cheeks. “I didn’t mean to do it.”
“I know, sweetie.” She wiped his cheek and he let her. Katsuki wanted to yell and scream, but he knew that wouldn’t do any good. Not now. “It’s only natural to feel a little possessive over someone you care about so much. It’s just a phase. Don’t worry about it. Izuku isn’t mad at you, and neither are we.”
Katsuki nodded, and then stood to go back to his room to get himself together.
It’s just a phase, Katsuki told himself later that night as he tried to fall asleep alone for the first time in forever.
It’s just a phase, Katsuki told himself as he and Izuku started sneaking into each other’s rooms at night to cuddle in bed, and then sneaking out before morning.
It’s just a phase, Katsuki tells himself, even though he knows it’s not.
It was just after that that Deku officially got registered as quirkless.
Maybe it was their new sleeping arrangements, or maybe deep down Katsuki thought of himself as better than people without quirks, or maybe it was a mixture of the two-- something dark and twisted that had been starting inside of Katsuki for Deku’s entire life. Maybe it was all of that, more than Katsuki could even identify. But with Deku’s official quirklessness, came a divide between the two brothers.
Katsuki got mean.
He’d always had a mean streak about him, always been aggressive and quick to fight, but never with Deku, never before.
Suddenly, making Deku cry was the best part of Katsuki’s day.
It’ll take years for Katsuki to figure out that he just wanted to be the most important person in Deku’s life. That he was desperately trying to cling onto a bond that he knew was slipping.
Katsuki got into UA and thrived. He made a few friends that he tolerated and worked hard. Deku continued to be a lame quirkless loser. For a few years, life was stagnant. But everything, really, everything changed when Deku started to show signs of being an omega.
“Izuku! You’re growing up!” Katsuki walks into the kitchen one morning to find his parents surrounding his brother, giving him hugs. Izuku’s cheeks are red and flushed. Katsuki slides himself into a chair at their kitchen table.
“What’s going on?” he asks.
“Izuku’s an omega!” Mitsuki tells him. “Both our boys are all grown up!”
“Yeah,” Katsuki says. “I guess that’s true.”
Deku refuses to meet his eyes.
As Deku grows, so does the divide between them. Katsuki has no idea how to fix it.
.
“Your little brother’s an omega, right?” one of his friends, the one with shitty hair, asks one day when he’s over studying in Katsuki’s bedroom.
“Yeah, what about it?”
Kirishima grins, his teeth are pointy like a shark’s but there’s something else about it that sends a shiver down Katsuki’s spine.
“He’s a cute little thing,” he starts.
Katsuki’s blood boils.
Kirishima continues, saying, “Wouldn’t mind teaching him what an alpha can do.”
Suddenly, Katsuki’s spinning around to face him, gripping him by his shirt collar and shaking him. Katsuki can feel his incisors elongating. He can feel his palms start to sweat. He rounds on Kirishima, getting up in his space as he bares his alpha fangs. “What did you say about my brother?” Katsuki growls.
Kirishima holds his hands up, his eyes wide and scared. “Hey, man! I didn’t mean anything by it!”
Katsuki pants, feral and furious. “Deku’s mine. Understand? My omega! Mine!”
“Shit, I got it! Calm down dude!”
Katsuki throws him back down onto his bed before he huffs and goes to sit on his desk. He takes a few deep breaths to calm himself. “Good,” he says. “Now let’s finish this damn project.”
Neither of them spot Deku spying on them through the crack in the door.
.
That night, Katsuki dreams.
In his dream, there’s a boy with their mother’s green hair and a face he’d recognize in a crowd of thousands. In his dream, Deku looks up at him with those big, green eyes of his, brimming with tears that threaten to spill. He asks, “Do you really want to be my alpha, Katsunii?”
In his dream, Katsuki can’t answer. Instead, he feels his jaws ache, his incisors elongating. He wants to claim, to mark Deku as his.
In his dream, that’s exactly what he does. Katsuki doesn’t know how it happens, but he’s moving inside of Deku, his brother, fucking into his sloppy wet hole again and again. Katsuki leans down, nuzzling into Deku’s scent glands before he opens his mouth and--
Katsuki wakes up in a pool of sweat with a faint burning smell in the air. He looks down to his hands and swears, as he notices that he’s singed his sheets in his sleep. He pats out the remaining embers and then flops onto his back, breathless.
It isn’t until then that he realizes that his pants are wet and sticky.
Fuck, he just came thinking about fucking Deku, about fucking his brother. He’s gonna be fucking sick. What kind of monster is he?
It’s then that he remembers the time he tried to claim Deku all of those years ago, back when Deku was in elementary school. What had his mom said? It’s only natural. It’s just hormones. That has to be it. There’s no way Katsuki would want to really fuck his brother.
He doesn’t sleep another minute that night. From then on, Katsuki avoids Deku like the plague. No longer does he try to make his brother cry or bully him.
He just can’t seem to shake that image of Deku from his dream, no matter what he does.
.
“You’re going to watch your brother for the weekend.” Katsuki’s mother tells him. Her finger is pointed at his chest. Katsuki can tell by the look on her face that there’s no getting out of this one.
He tries anyway, shouting, “What! Why?”
“Your mother and I are going away for the weekend. You’re old enough to stay home alone, but Izuku’s a little too young. So instead of hiring a babysitter that will drive you crazy and cost a fortune, we just decided that you’d watch him instead.”
Katsuki rolls his eyes. His mother isn’t wrong. He’d much rather have to deal with his loser brat brother than have some annoying babysitter running around his house. He groans, making a real show of how annoyed he is at all of this.
His mother glares. “You’re watching him. That’s final. It’ll barely be any work. You know all the kid wants to do is stay in his room and watch All Might videos anyway.”
Katsuki throws his hands up and then goes and stalks off to his room, already angry about the days to come.
.
Deku manages not to cry when their parents leave which surprises the hell out of Katsuki. They head back inside and Katsuki grabs the pizza money that was left for them on the counter. “Hey! What do you want on your pizza, nerd?”
Deku makes a face and then shakes his head. “Nothing. I’m not feeling too well.”
Katsuki rolls his eyes. “You better not be getting sick. I won’t clean up your puke.”
He waits for Deku’s response, but his brother’s already gone.
Katsuki screws up his face. Something about this feels off. He brushes the worry away and orders the kind of pizza he wants. At least now he’ll get some alone time.
He goes the rest of the night without seeing Deku. About an hour before he goes to bed, Katsuki can’t help but shake a nervous feeling. Something’s wrong with Deku, with his brother. He groans and runs a hand down his face, annoyed at himself. He could have just chilled and gone back to his room to rub one out and go to sleep, but nope, of course not. Now he’s climbing up the stairs, filled with a terrible sinking feeling and a hundred different worries about being without his parents. What if Deku needs to go to the hospital? What if he’s really sick?
He makes it halfway up the stairs before the scent of omega slick hits him in the face so hard he stops in place.
“Oh fuck,” Katsuki says, taking in a huge gulp of air. That’s why Deku was feeling sick earlier. He’s in fucking heat.
Katsuki doesn’t know if he should try and hold his breath and run away or not, but with every moment he stays still, he can feel his alpha instincts taking over. There’s an omega in heat. His omega, his mind supplies him. With every breath he takes, Katsuki feels himself losing control.
Since the first day he was born, Izuku’s been his. Now is not any different.
Katsuki doesn’t bother knocking, instead just shoving open the door to Izuku’s room.
“Katsunii!” Izuku whimpers. He’s all tangled up in his sheets and from here, Katsuki can smell the hint of heat sickness coming off of him. “Help me. W-what’s happening?”
Katsuki’s over to him in a second, climbing up onto Izuku’s bed and bringing his brother onto his lap, something he hasn’t done in years.
Suddenly, in that moment, there are no quirks separating them. There’s no divide made out of things Katsuki could never place or name tearing them apart. There’s just Katsuki and Izuku, tangled up together, just as they have a million times before. Inside Katsuki’s soul, he swears he feels something that he’d been missing slide into place.
“Shh,” he coos at Izuku, rubbing his sweaty hair off of his forehead. “You’re in heat. I can smell you from the stairs.”
Izuku gives a little groan. “It hurts.”
“I know it does, baby,” Katsuki tells him, the pet name falling naturally from his mouth. He holds Izuku a little tighter than he normally would, squeezing him extra hard for all those years he didn’t.
“Katsunii is an alpha.” Katsuki can tell that Deku’s started crying, his hands scrabble to hold Katsuki’s shirt. “You smell so good.”
Katsuki lets out a groan at that, and it’s then that he realizes his cock is so hard it aches.
This. This is what Katsuki’s been missing all along, what they’ve been missing.
“Do you think so?” He asks Izuku, because for as much as Katsuki knows he wants this, he won’t do anything if Izuku doesn’t want it. He can’t hurt his brother like that. He refuses to.
“Yes,” Izuku answers. His smaller body is wiggling along Katsuki’s. He’s probably so wet, so desperate. “Katsunii can help me. Please? I feel weird. I want--” He breaks off into pants.
“What do you want, Deku? Do you want me to touch you?”
“Please!” Deku flat out moans, his back arching towards Katsuki. “Please, I need you, Katsunii.”
That’s more than enough for Katsuki. His head is swimming, so caught up in Deku and everything about right now, everything that he’s missed for so long.
Katsuki brings his hand down Deku’s body, down to where he knows his brother needs it. He cups his little cocklet-- hard and straining against his underwear. “Do you want me to touch you here?”
“Oh,” Izuku moans, “Yes, yes that feels so good.”
Katsuki brings his hand lower, down to Deku’s cunt. He’s soaking wet, his body begging for an alpha to slide inside him and breed him. “What about here, Deku?”
“Katsunii!” Deku screams. “Please! Please! Touch me there! Go inside me!” Deku’s voice cracks and his head lolls, his faint chants of ‘please, please’ barely loud enough to hit Katsuki’s ears. His hips roll up into Katsuki’s touch.
“Damnit, Deku,” Katsuki has to close his eyes and take a minute. This is his first time, too. “I’ll take care of you. I won’t leave you again.”
And with that, Katsuki crosses a line he knows he shouldn’t cross, does something that he doesn’t think he and Deku will ever come back from. He kisses him.
Deku’s mouth is hot and desperate as it moves against his, and his body comes up close to Katsuki’s, throwing a leg over his hip. Katsuki cups his jaw, trying to slow his eager brother down. Katsuki’s kissed someone before, so he knows how to do it. He knows just how to flick his tongue against Deku’s, how to part his brother’s lips and ease his way inside. Deku makes a noise against his mouth-- a pleading, whining sort of noise that shoots straight down to Katsuki’s cock, and he swears that he wants to bottle it up and save it for later.
Deku’s tongue is soft against his own and Katsuki moans into his mouth, feeling far too much to keep any of his noises inside. It’s just them here. There’s no one to stop them or to find out about what they’re doing.
“I want you inside of me, Katsunii,” Deku moans, his voice shaking. “Please?”
“I will, baby. I will. Gonna make you mine, Deku.”
Deku’s body writhes at his words, his hips rocking on Katsuki’s thigh in desperation. He’s so wet he’s soaked through his sleep shorts. Katsuki peels them off, throwing them onto the floor without a care. Katsuki lays his brother out on his bed like the precious thing that he is. Deku’s eyes stare up at him, big and shining with unshed tears.
“Katsunii,” he whimpers.
Katsuki thinks that he must be so scared. He remembers how horrible his first rut was. Katsuki leans down and places a gentle kiss on Deku’s forehead. He tangles their hands together, kissing Deku’s lips next. “I’m going to taste you, and then I’m going to fuck you. I’m going to make all your pain go away.”
Izuku blinks and tears fall down his flushed, freckled cheeks.
Katsuki kisses his way down Deku’s body, stopping when he gets to his chest to lift his shirt up. His tits are so cute, so tiny and perfect for a male omega. Katsuki sucks one of his puffy nipples into his mouth, relishing in the way that Deku cries out his name. His hands come to grip Katsuki by the hair. Katsuki focuses his attentions there, suckling on Deku’s nipple for a moment before switching sides. Soon Deku’s shivering from overstimulation and Katsuki keeps working his way down.
Deku’s cute little cocklet is lying hard on his belly. Katsuki practically drools when he sees it, but there’s something he wants to taste even more. He gets himself eye-level with Deku’s sopping wet cunt. He takes a big breath in, taking in all of Deku’s sweet, omega scent. Then he dives in for a taste, licking a long stripe up Deku’s slit. Katsuki moans at the delicious taste of Deku’s slick on his tongue. He licks around Deku’s hole greedily before he brings Deku’s cocklet down to his mouth to suck it inside.
Deku screams, coming immediately, slick gushing from his hole as his cocklet shoots helplessly on Katsuki’s tongue. “Katsunii, Katsunii!” Deku cries. When Katsuki pulls off of his cock, his eyes are wet, his lips bitten red.
Katsuki purrs, a sound he didn’t even know he could make until right this very moment. “My omega, so pretty. I’m going to fuck you now, omega.”
Deku nods weakly, rolling over and presenting himself to his alpha.
His hole is slick, begging for Katsuki’s cock. He can’t let his omega wait for much longer.
Katsuki grabs his brother by the hips, and then sinks his cock inside of him.
He goes slowly, letting Deku’s virgin cunt get used to the intrusion. He moans as his cock is swallowed up by velvety heat.
“So damn tight around me, Deku, holy shit.” Katsuki loses himself in the feeling, and it feels like forever until he finally bottoms out inside of his brother. As soon as he does, he feels Deku tighten up so much that it actually hurts. He’s coming again, just from having Katsuki inside of him. Katsuki starts purring again, his inner alpha unbelievably thrilled that he’s causing this kind of reaction in his brother, in his omega.
“You okay?” He asks.
“Move, Katsunii!” Deku moans. He starts to writhe against Katsuki. “I need it.”
Fuck. How is Katsuki supposed to say no to that?
He starts to move inside Deku then, sliding his thick cock in and out of Deku’s greedy hole. He fucks Deku carefully at first, treating him like the precious thing that he is. He grips Deku by the hip and slides his cock in and out, moaning at the feeling of it all.
Then Deku says, “Harder, please, Katsunii,” and Katsuki gets so overwhelmed he swears he whites out for a moment.
“Fuck!” he swears, slamming his cock into Deku without really meaning to, but the noise that Deku makes in return is worth it. He gives in then, fucking into Deku how he really wants to-- hard and fast. He grips Deku with both hands, uncaring that his fingers will surely leave bruises in their wake, and shoves his cock as deep into Deku’s pussy as he can.
Katsuki fucks his brother-- his omega -- then, really fucks him. The sound of their skin slapping together is the loudest thing in the room, spare their occasional cries and moans.
Deku’s cunt squeezes his cock so tight, he knows he’s not going to last. “Feel so good, Deku, you’re gonna make me knot you.” He breaks off into a moan, the mere thought of knotting his baby brother bringing him that much closer. “Can I knot you?”
“Yes!” Deku wails. “Please! Alpha!”
It’s that-- being called alpha, Deku’s alpha-- that gets him. His knot is already swollen but it’s Deku’s voice that makes it pop. He shoves into Deku’s cunt once, twice before they’re locked fully together. Katsuki fills his brother up with his seed, moaning along with Deku’s whines and cries.
“Baby…” he mumbles against Deku’s skin. “My baby.”
Deku comes again, squeezing around Katsuki’s knot immediately at the feeling of being filled finally by an alpha.
He immediately falls limp against the bed.
Katsuki’s drawn to him like a moth to a flame, laying next to him, kissing him everywhere he can reach. “I’m never going to leave you again, Deku.”
“You promise?” Deku asks. His words are slurred.
Katsuki puts a hand against his belly, full of his seed. “I do,” He says. “I do.”
He holds his brother close and says, “I’ll take care of you from now on.”
Deku hums happily, and Katsuki knows he believes him.
.
“You two seem to have gotten close again, what happened?” His mother asks him a few days after they get back. Her red eyes stare into Katsuki’s own.
“We made up,” Katsuki tells her with a shrug because in a way, it’s true. Deku’s heat was gone by the time their parents got home, but ever since then, they’ve been inseparable once more. They do their homework together, watch tv together, they even eat meals without glaring at each other.
Katsuki knows that it was just the urges that went with Deku’s heat that brought them together. He knows it was wrong, but he doesn’t regret it. It got him his brother back after years of not even knowing he was missing him.
Katsuki knows that it can never, ever happen again. He has to tell himself that more than he should.
.
He doesn’t even stir until Deku’s all the way in his bed.
“Deku what-”
“Katsunii,” He whispers. It’s the middle of the night. Katsuki blinks himself awake and notices that Deku’s crying.
“What is it? What’s wrong? Did you have a bad dream?”
“No.” Deku shakes his head. “I know it’s bad and that we’re not supposed to, but I want you, Katsunii. I need you.”
“Deku--”
“Please!” Deku puts his hands on Katsuki’s face, his eyes wide and pleading. “I want to be your omega forever and ever. I need it again. Make me yours.”
“Deku, baby, we can’t. It’s--” Katsuki breaks off. His own resolve is fading and fading fast.
“Katsunii, mate me.”
That’s it, that’s all it takes.
Katsuki purrs low in his throat at the chance of being able to do something that he’s wished he could do for years and years. He gently rolls Deku onto his back, laying him out on his blankets and pillows. “You wanna be mine?”
“I do.” Deku’s eyes are so big, so beautiful. Katsuki wastes no time, pulling off Deku’s little sleep shorts. He’s not even wearing underwear underneath them. His pussy is sopping wet, the smell of his slick filling the air. The image of Deku fingering his cunt and thinking about him floods his mind. He gets hard so fast he’s dizzy.
“You wanna belong to your brother, Deku?”
One single word seals the fate.
“Yes.”
His lips are on Deku’s then and the kiss is immediately hot and open-mouthed. Deku opens his lips and lets Katsuki inside. He’s learned from all the kisses they shared last weekend, and now he kisses with a confidence that drives Katsuki crazy. It’s his tongue that slides into his alpha’s mouth, demanding but soft-- needy.
Katsuki wastes no time making his omega feel good. He knows that their parents are sleeping just one room over. One of them could get up and notice that Deku’s not in his bed and then it’d all be over. He puts his hand down to part Deku’s thighs and slides through the wetness between them. He brushes his wet fingers against Deku’s cocklet before pushing one inside of his hole. Deku moans into their kiss, obviously trying to hold himself back but failing.
“Shh,” Katsuki tells him. “We have to be quiet.”
Deku’s hole gives him more resistance now that he’s not in heat, so it takes a few minutes for one finger to become two but they get there. Deku comes as soon as Katsuki slides the second finger in, a little squeak and a gush of slick leaving him. Katsuki feels too big for his skin, too powerful just knowing that he is able to take his perfect, precious baby brother apart like this.
Deku’s a shivering mess by the time Katsuki works a third finger into his cunt. His fingers dig into Katsuki’s biceps.
“Katsunii, please put it in.” His voice sounds small and shaky and so much like Deku that Katsuki thinks he might lose his mind.
Katsuki swears, practically ripping off his sleep pants and underwear. His cock is already hard, it’s been hard. It aches as he wraps his hand around it, pumping his quickly up and down as he gives himself a few strokes with his slick-covered fingers. He presses his cock up against Deku’s slick hole, teasing.
“Come on, do it,” Deku whispers, sounding completely overwhelmed. Katsuki does exactly as he is told.
He presses his cock inside of Deku, breathing out heavily through his nose. In the moonlight, he can see that Deku’s eyes roll to the back of his head and he brings a hand up to his mouth to quiet his moans. Katsuki can still hear the ghost of them, and it’s just enough to snap the last thread of control that Katsuki was clinging onto.
He lets out a feral growl into the night, gripping Deku’s hips and lifting him so he can get a better angle, get in deeper, fuck harder. He lets his instincts guide him, fucking into his omega so hard that his body moves up and up and up Katsuki’s bed. He’s thankful he doesn’t have a headboard, or else it would be slamming against the wall. Every few strokes, Katsuki has to grab Deku and pull him back down onto his cock. Deku writhes, his nails scratching against Katsuki’s arms whenever he can reach them.
Deku’s body suddenly goes rigid and his pussy clams tightly around Katsuki’s cock, squeezing it so hard that Katsuki has to stop moving in order to keep from coming before he wants to.
Katsuki gives him a moment to move his hips on Katsuki’s cock and ride out his orgasm before he starts to thrust again.
“You still want me to claim you, omega?” Katsuki asks.
Deku nods frantically, panting, “Yes, yes, yes.”
“Fuck,” Katsuki moans. He can feel himself getting close and knows that if he wants to mate his brother, he has to do it now. He thrusts into Deku’s wet heat once, twice before he leans down to the scent gland on his neck and sinks his teeth into the skin. Blood fills his mouth and their combined scents fill the air. Katsuki’s knot immediately pops as his brain swims with his brand new bond. Katsuki releases him then, knowing that Deku is properly mated and claimed. Deku lets out a moan at the overwhelming feeling, and Katsuki slams their lips together to keep him quiet. His body jerks as he shakes through another orgasm so quickly after the last.
Katsuki kisses Deku through the worst of his overstimulation, sweetly bringing his brother down from his peak. Deku shakes and shakes, and Katsuki only pulls back from his lips to lap at the blood that drips from his bite.
Katsuki cradles Deku in his arms the best that he can while they’re still tied together. He kisses his forehead gently, rubbing soothing strokes up and down his back. What they’ve done on this night can never be reversed.
Katsuki doesn’t think he’s ever been happier in his entire life.
Deku is finally his.
“I love you, Katsunii,” Deku says.
Katsuki’s heart soars. “I love you too.”
In his mind, everything is exactly how it’s supposed to be.