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They don’t know what the quirk does at first.
There’s nothing on the record about the guy who hit Shouto, and even the damn nerd shrugs apologetically when Katsuki asks.
When he gets to the scene, Katsuki is just happy Shouto’s in one piece, even if—
Well, he does cling to Katsuki a lot more than he normally would in public, pouts about not letting go of his hand even while the medics are trying to look him over, and when they get home, he’s plastered to Katsuki’s side, eyes big and wide and pleading when Katsuki pulls away even half an inch.
And, speaking of inches—
Well, Shouto’s real eager when it comes to that, too, straddling him in the damn hallway and spitting on his hand just to stroke Katsuki’s cock like that, running his mouth about how big it is and how warm it feels and how much he wants it inside before pressing the tip where he’s wettest and sinking down in one go, Katsuki’s hands on his hips not much of a deterrent.
By the time Shouto’s done bouncing on it like a good boy, Katsuki is panting and sweaty under him.
“Think it was a sex quirk?” he asks. “You don’t even get like this during your heats sometimes.”
Shouto shrugs. “Dunno,” he says. “Just—felt empty inside. Needed you so bad, alpha.”
Katsuki strokes his sweat-soaked bangs off his forehead. “You have me,” he says. “Don’t even gotta ask.”
“I like asking,” Shouto says, nuzzling into Katsuki’s palm when it slides down to cup his cheek. “You always give me just what I need.”
And—
Katsuki would argue that what just happened was a lot more about Shouto taking exactly what he wanted than anything else, but he’s never minded that either. He’s never minded anything, with Shouto.
Not even—
“You know you just get away with it because you’re real pretty, don’t you?”
Shouto laughs. It’s Katsuki’s favorite sound in the whole world, Shouto’s happiness spilling out of that pretty mouth. “Yeah,” he says, nose scrunching. “I had a feeling.”
—not even the tough parts. Because looking at Shouto softens them into simple things as easy as breathing.
So. Katsuki doesn’t mind. Being what Shouto needs. Giving him exactly what he wants even while tears spill down his warm cheeks and he whines about not being full enough with Katsuki’s knot swelling in him.
But he does think that’s the worst of it as far as the quirk goes.
Shouto seems to settle too, mostly content with Katsuki holding his hand while he cooks dinner. When they go to bed, he makes a little nest with Katsuki’s clothes, and then pulls him in, and they sleep like that, pressed closecloseclose.
…
Katsuki wakes up to the distinct scent of omega heat. Of Shouto’s heat.
Except, Shouto’s not—
Shouto’s a lot smaller than he was just a day ago, his tiny body barely managing to straddle Katsuki’s hips, Katsuki’s borrowed sweater huge on him, and he’s tugging on it like he’s trying to hide even while he wiggles his hips, all slick between his soft thighs.
“Shouto?” Katsuki says, voice still rough with sleep.
“Alpha,” Shouto whines. He sounds like he’s going to cry. “Alpha, it really hurts.”
He’s burning up to the touch, skin warm in that sickly away it only gets during an unattended heat.
“Where does it hurt?” Katsuki asks, thinking of the quirk, thinking of how nobody knew shit about it, how—Shouto looks too young for a presentation heat, but the way he smells—
Shouto’s cheeks redden. “It’s dirty,” he says. “I don’t think I’m supposed to tell you. Daddy would be mad.”
Right, there’s that too.
Shouto never told him anything about when he presented, and Katsuki always assumed it was unpleasant for him, but—Endeavor probably didn’t want an omega son, did he? Not much use as a perfect little soldier when the mere suggestion of an alpha command will get you slick between your thighs.
Omega heroes tend to handle rescue and cleanup.
Of course Shouto was too damn precious for that, though.
Katsuki tries his best to soften his edges for Shouto’s sake. Now isn’t the time for anger. “It’s okay, sweetheart,” he says, as evenly as he can manage. “You can tell your alpha anything. You know that, right?”
“My alpha?” Shouto repeats, wide eyed. “You’re my alpha? Is that why you smell so good?” Then, quieter, “Is—is that why it’s all sticky between my thighs?”
Fuck, that’s—
Would it even fit while Shouto’s like this?
“It’s okay,” Katsuki repeats. “It’s just a heat, princess.”
Shouto’s quirk flares up, a thin layer of ice creeping up where he’s clutching at Katsuki’s forearm. “It hurts down there,” he whispers, like he’s sharing a shameful secret. “My—my princess parts hurt.”
“Can I see?” he asks. “Is it okay if alpha touches you, pretty omega?”
Shouto bites at the swell of his lower lip. “Can you be gentle?” he asks. “It really does hurt. I know omegas aren’t good for much, but please don’t make it worse.”
Oh, Katsuki is gonna fucking kill Endeavor, actually.
Later, though. Right now he’s got a lapful of needy omega to take care of.
“Won’t hurt you,” he promises. “Never gonna hurt you, sweetheart.”
Shouto smiles at him. “Can you use your mouth?” he asks. “I think—I think that’s what my omega wants.”
“Gonna have to let me see for that,” Katsuki points out, gesturing to where Shouto is still clutching at the fabric of his sweater to keep it in place with a little hand balled up in it. “C’mon, I bet you’re real pretty.”
“Alpha thinks I’m pretty?” Shouto asks.
Katsuki nods. “The prettiest,” he says. “Prettiest omega in the whole world.”
“Oh,” Shouto says softly. “I think I like you.”
And then he’s scooting off Katsuki so he can lie back in the pillows and part his thighs, his little pussy all pink and wet between them.
The selfish part of Katsuki wants to try and sink his knot right in. Of course it does. His chosen mate is presenting. Any good alpha would give him a knot.
Instead, he parts Shouto’s labia with two fingers, keeping his folds spread so he can look at the way his clit twitches when he blows air on it.
“Mean,” Shouto whines. “You’re being mean, alpha.”
“Not mean,” Katsuki says, kissing Shouto’s puffy clit before dragging his tongue over it. “Not mean at all, just wanna look at you. Because you’re so pretty. My pretty little princess, yeah?”
Shouto nods eagerly, keening at each swipe of Katsuki’s tongue. “Feels weird,” he says. “Alpha, I think I have to pee.”
In response, Katsuki’s licks turn firmer, teasing at Shouto’s clit with the tip of his tongue until Shouto’s thighs are twitching, and then his perfect little omega squirts all over him.
“That’s a good boy,” Katsuki says, a hand on Shouto’s thigh to hold him open. “Taste so good, baby.”
Eventually, Shouto tries to shove him away. “Too much,” he whines. “Empty. Feels empty inside, alpha.”
Katsuki pushes a finger in him. Shouto’s so soft and warm inside, and Katsuki’s alpha is growling about knot and breed and—and really, just the thought of that tiny little pussy stretching enough to take the swell of his knot, Shouto’s belly bulging with come more than it ever has before, has Katsuki nearly feral with want.
“Can’t put a knot here,” he coos instead, pressing a second finger carefully inside. “You’re going to break, sweetheart.”
Shouto shakes his head stubbornly. “I won’t,” he whines. “I won’t, because—because I like you a lot.”
God, just two fingers already feels like too much.
Katsuki’s cock is fuckin’ leaking in his underwear. He pictures it, the face Shouto would make if he pressed the tip of his cock to his pussy, barely stretched enough even with Katsuki rubbing him from the inside like this, teeth digging into his lower lip, his little whines more pleasure than anything even with the impossible stretch. Katsuki would go slow, make it good for him.
Maybe—maybe just to take the edge off a little?
He tugs his sweatpants off, a hand wrapping around his cock.
“It’s big,” Shouto says, awed. “Does it hurt?”
And then he’s—
And then he’s closing his parted thighs all shy and crawling on his hands and knees just to get closer and place a delicate hand right above Katsuki’s. It doesn’t even wrap around halfway.
“Knot’s already swelling,” Katsuki says. “But it’s okay, already told you I’m not going to hurt you.”
Shouto looks contemplative. Then, he leans forward, and drags a thumb over the leaking head. “You kissed it better for me,” he says. “I want to kiss it better for you too.”
Stubborn little thing that he is, he doesn’t even wait for permission, tiny mouth smeared with Katsuki’s come while he kisses and kisses and kisses over the swollen head, sticking his tongue out to lick at the slit like he’s curious.
“Fuck,” Katsuki hisses. “You’re gonna fucking kill me.”
“That’s a bad word,” Shouto says, pulling back and wiping his mouth with the back of his hand. “I don’t think you’re supposed to say it.”
Katsuki laughs. He’s probably not supposed to do a lot of other stuff too. “Yeah,” he says, strained. “That’s fair.”
Shouto looks at him for a bit, mouth pursed like he’s trying to make up his mind on something. “Can you put it in now?” he says. “It burns inside.”
And Katsuki really, truly, absolutely shouldn’t, but—
Well, it’s Shouto.
It’s Shouto and Shouto needs him and Katsuki isn’t his piece of shit dad, he’s not going to hurt him, but—Shouto wants it, right? His omega wants comfort through a heat, wants the stretch of Katsuki’s knot in his pussy, and Katsuki is supposed to give it to him. That’s how it always goes.
“Okay,” he says, pressing a kiss to Shouto’s forehead, “but tell me if it hurts and we’ll stop, alright?”
Shouto nods. “Thank you,” he says. “You’re a good alpha.”
Katsuki gets him comfortable on his back again, licks at him some more, slicks up his own cock with the wetness dripping out of his perfect little omega, and then the head of his cock is pressing inside. Shouto takes it. Shouto takes it, thighs parting wider and little keens turning more and more high pitched until Katsuki’s just staring at how wide the head of his cock stretches Shouto’s pussy.
“More,” he whines. “Alpha, I can take more.”
So Katsuki listens, and pushes in deeper. He’s about halfway inside when Shouto’s whines turn into something a little pained, and suddenly Katsuki realizes—
Shouto can’t fit more. No, because Katsuki is already as deep as he’ll go, pressed snug against his cervix.
“Feels good when you, ah, when you rub there from the inside,” Shouto says. “Feels—ah, full.”
Katsuki doesn’t fuck him, not really. But he does rub his cock against that spot until Shouto’s trying uselessly to grab at his shoulders, and then everything is significantly softer and wetter and tighter, and—somehow, Katsuki’s cock pushes in deeper.
“Oh,” says Shouto, mouth parted while he clenches tighttighttight around Katsuki.
It doesn’t take a lot for Katsuki to come with Shouto squeezing him like that, his pleasure seemingly never-ending, the swell of Katsuki’s knot just barely managing to fit inside until he can barely rub at Shouto’s clit to ease him through it from the stretch.
His belly’s bulging from how much Katsuki comes. Shouto pokes at it curiously. “Does this mean I’m going to be a mom?” he says, and—
And, sure, they talk about it even when Shouto isn’t digging his nails into Katsuki’s back begging for it through a heat, they both want it, Katsuki knows that, but—
This Shouto is so small.
He’s looking at Katsuki with something hopeful in his pretty eyes, though, eyelashes clumped together with tears just barely spilling over, so.
“Sure,” Katsuki says. “If you want.”
Shouto smiles at him. “I think I do if it’s with you,” he says. “I’d like to give you pups. You’re a good alpha. I like that I’m yours.”
Katsuki doesn’t say anything to that, but he does pull Shouto close. For now, he can hold him. For now, he can make sure he’s safe.
It’s enough. It has to be.
…
When Shouto goes back to normal, he spends a full day not being able to look at Katsuki in the eye.
Eventually, right in the middle of dinner, he goes from wordlessly slurping his soba to looking up just to say, “Do you hate me now?”
Katsuki stills. “What the fuck?”
“Oh,” Shouto says. “You know, because I made you touch me when I was—like that.”
“You’re my omega,” Katsuki says, even as uncomfortable heat prickles at his skin. “Of course I was gonna take care of you. Besides, I didn’t—didn’t exactly mind, you know.”
Shouto’s eyes widen. “Oh,” he says. Then, “Was it because I was smaller?”
Katsuki shrugs. “It was because it was you,” he says. “It was you, and you needed me, and—and yeah, I guess I liked having you that vulnerable just—just at the primal level or some shit, but it’s not—I don’t want that. I just want you.”
“That’s sweet,” Shouto says, mouth twitching. “You really are a good alpha.”
And—
“You remember?” Katsuki asks. “What you said?”
“A little?” Shouto says. “I mostly remember how safe I felt, and how much I wanted—well, you.”
It does make the stupidly alpha part of Katsuki want to puff out his chest. Doing a good job at taking care of his omega. Making him feel good and keeping him safe and—well, filling him up well enough too.
“Yeah?” Katsuki asks. “Just me, huh?”
“Just you,” Shouto repeats. “Maybe it wouldn’t hurt to have a few pups running around once things calm down a bit, though. What do you say?”
Katsuki leans over the table just to pull Shouto in with a hand on the back of his neck and kiss his forehead. “Whatever you want, princess,” he says. “As long as you’re happy.”
And Shouto—
Shouto whispers, “I am,” like he wants Katsuki to know it most of all. “I’m really happy because of you, alpha.”
(Yeah, Katsuki is too. Of course he is. He’s got Shouto.)