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Promised Through Sin

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“Katsu-nii I wanna plaaaaay.” Izuku whines, squirming in his lap.

“Well you’re stuck right here until I finish my assignment.” Katsuki says, planting a hand on Izuku’s stomach to hold him in place. “Don’t complain, you know how busy I am? You’re lucky I take the time out of my day to hang out with you.” A lie, for sure. It’s really no skin off Katsuki’s back to babysit the little nerd while he does his homework. Not that he’ll admit it. He likes having Izuku think he’s doing him a favor just letting him be in his presence.

“I – I don’t like waiting.” Izuku complains, and Katsuki doesn’t have to see his face to picture that perfect pout he’s got going. “S’boring.” He mumbles, but he’s not fighting to get off and Katsuki knows the kid will wait all fucking day if he has to, if Katsuki orders him to. He catalogues that idea for another day.

“Too bad. Shoulda thought of that before you came sniffing for attention.” He says, leaning forward to sink his teeth into the nape of Izuku’s neck. The younger boy tenses and then goes slack against him, a soft little noise escaping him. God, Katsuki loves his noises, the soft vulnerable sounds go straight to his dick every time. “Now you gotta wait on my lap ‘till I’m ready for ya.” He says, licking at his possessive mark. It’s the kind of shit he can pass off as disciplinary, if he needs to. Not that anyone is ever looking that close, anyway.

Katsuki is probably the only one who’s got time for him these days. Between the kid’s dad being gone and his mom working all the damn time, it’s usually Katsuki’s job as their neighbor to babysit him. Usually, he just sticks the kid downstairs in front of the TV while he works on his own shit. Auntie Inko doesn’t expect much from him. Just someone to keep an eye out and maybe shove some dinner in him if it gets too late.

Today, though, Izuku had come upstairs, hesitant and red-faced but clearly looking for affection. Katsuki’s feeling mischievous, so instead of laying the kid down on his bed and kissing all over his soft little body, he’d picked him right up and sat him on his lap, making him wait a minute before he tucked in this tiny meal.

He’s only got a couple more questions to puzzle through anyway, and having Izuku’s warmth pressed up against him is as good a motivation as anything. Katsuki leans forward, resting his chin on Izuku’s boney shoulder to get a better look at the desk in front of them.

“Katsu-niiiiiii.” Izuku whines, shifting around even more and puffing out his cheeks where they’re pressed against Katsuki’s own.

“Mhm.” Katsuki says, rubbing at his stomach before stealthily sneaking his hand underneath the thin t-shirt and skimming his fingers against all the skin he can reach. Izuku’s so fucking smooth and Katsuki thinks of how much of a total fucking pervert he is, that this is the type of shit that gets him going now. He thinks fondly back to the days before Izuku was even goddamn born and he’d jerked it in peace to normal porn like everybody else.

Thirteen and thriving, honestly. He wonders when he’d started to see this sticky, annoying little creature as something he could have, something that was his.

Somewhere in between Izuku’s fifth and six birthday, probably. When he’d looked up at Katsuki with those big doe eyes and proclaimed that he loved Katsuki the most, and he knew right that second he was absolutely and completely fucked.

But Izuku, as always and only just for him, has always been so eager and willing to do anything and everything he asks. Sure, he might put up a token struggle, but Katsuki’s practically claimed him at this point, and there’s no way a young, budding omega would be able to deny him anything, especially with the way Katsuki’s constantly rubbing his scent all over him.

He’s glad, distantly, that they spend so much fucking time together, Izuku always tangled up in his space. He doesn’t know how he’d explain his scent all over the kid, otherwise. They all probably have their theories about his intent, but intent and action are two different things.

“Tickles...” Izuku mumbles, pulling him out of his thoughts to focus on the idle patterns he’d been tracing around the other’s small belly-button. Katsuki smirks to himself, digging his fingers in with increased force before smoothing them up to his chest, pulling the shirt up higher.

“Does it tickle here too?” Katsuki flicks at one of his pert little nipples, pulling on it sharply before rolling it between his fingers. Izuku jerks on his lap, back arching into the touch just enough to let Katsuki knows it absolutely does not tickle.

“Kacchan!” Izuku complains, trying to sound serious and instead just causing Katsuki to smother a laugh into his slender neck.

“Yeah?” Katsuki mumbles into his skin, pressing a few noisy kisses there as he switches his attention to Izuku’s other nipple. He wants to see, though, and who is he fooling? Now that he’s got a hand up the kid’s shirt there’s no way he’s getting anything else done. Not when Izuku’s breathing is starting to get a little shallow and Katsuki can smell his honey-milk scent spread across the room and deepen.

It’s pre-bloom and so innocently sinful that it goes right to Katsuki’s dick, filling it up against the snug fit of Izuku’s ass and thinks, fuck it. He can finish his homework later. After this cute little thing is out of his home, not stuffing his head full of cotton and desire just by existing in the same space.

He can smell a few other things too, the leftovers in the oven, the dirty shirt he’d forgotten behind his bed on his last laundry run. His senses are heightened, over-sensitive and sharp with the little one he’s got in his lap.

“Gonna take this off, kay?” Katsuki says, pulling at the rucked up shirt. “Lift your arms, Deku.” And he does without hesitation. Katsuki chucks the shirt, peeking over Izuku’s shoulder to look at the petite rosy nubs standing at attention.

“Look at you.” Katsuki mumbles, bringing both hands around to pinch and pull and explore. “You’re already all hard up here for me.”

“N-nipples don’t get hard Katsu-nii.” Izuku says. Katsuki gives both of the pink peaks a particularly harsh tug. Izuku cries out just the way Katsuki expects him too, pressing his lithe body up against him. Katsuki is powerless to stop the slow grind of his hips against that round little ass.

“Oh yeah? Where’d you learn that? Looks like yours are getting pretty hard for me.”

“You told me!”

“Did I?” Katsuki says absently, tips of his fingers pressing against each rib on the kid. He’s not too scrawny, sharp in some places but with a layer of baby fat still lingering on others. Katsuki can’t get enough of it. It’s like snuggling into his own personal squishy chew toy – one that he could have spread out under him right now instead of kicking up a fuss on his lap.

“Guess what?” Katsuki says, own thoughts spurring him into action. “We can play now.”  He gets his arms around the tiny body securely before he stands, Izuku letting out a high pitch squeal of excitement in lieu of an answer, hands raised in the air as he yells loud enough for the neighbors to hear.

He twists in his grip until they’re face to face, a little ray of sunshine directed right at him. Katsuki looks at those bright green eyes, that constellation of freckles, and thinks, looking at this work of art, that any other conclusion for them would have been impossible.

Izuku’s been important to him since the first time he held him in his arms, there’s no way that Katsuki could have stopped this from happening. At least, that’s what he tells himself so he doesn’t feel like a dirty pervert for laying his hands on the kid.

“Put me down!” Izuku yells, all smiles as he wiggles around in Katsuki’s grasp. He’s not really trying to break free, which is made all the more clear when Katsuki swings him around and he whoops. It’s cute and wholesome and Katsuki doesn’t know why his cock twitches in interest just at the sight of him.

In two big strides he’s standing by his mattress, Izuku looking down at the bed almost expectantly before Katsuki drops him onto it.

He lands with a quiet pomf, blinking some of the shock out of his dewy green eyes as he reaches out for Katsuki. The older boy is happy to oblige, lowering one knee onto the bed and hovering over him; letting Izuku snag his grabby little hands in his shirt and pull him closer.

“Katsu-nii you smell good.” Izuku says as he scrunches his nose and lets his mouth drop open to scent him better. Katsuki’s eyes rake over every inch of him, from the freckled cheeks to the equally spotty chest. Sometimes he wonders how he managed to get his hands on something so cute, so perfect, but he tries not to count his blessings too hard.

Katsuki smiles down at him, something a bit too fond squeezing at his chest. Man, he really is fucked.

“S’cuz I’m supposed to smell good to you, dumbass.” Katsuki says, leaning down to bite at one delicate, freckled cheek. Izuku pushes ineffectually at him, grinning as Katsuki gnaws on his cheek, before moving to nip at his nose. “I’m your alpha.”

The kid always goes wide eyed when he says it, looking up at him like he’d hung the fucking moon. Even young, even before he’s fully ripe, there’s an instinctual part of the kid that is clearly preening at being claimed. Katsuki tries not to feel sleazy at the idea that it’s exactly that part of Izuku that had been so easy to win over.

“Katsu-nii is mine.” He says, awe saturating his tone.

“Yeah and you’re mine.” Katsuki responds, nosing at the delicate flesh between Izuku’s jaw and his collarbone. He wants to sink his canines in so badly it’s a physical ache, but he knows he has to hold back. His scent he can explain away. A mating mark? Not so much. He satisfies himself with licking a wet stripe all the way down Izuku’s neck.

Izuku just laughs at this, squirming around a bit more like it’s gross. Katsuki supposes it is, the kid’s too young to understand why Katsuki does half of the things he does. Everything’s just feel-good playtime to him. Katsuki has years to train him up to his sexual awakening. But for now, he’ll settle for stimulating the kid manually.

He drags his tongue lower, over Izuku’s soft chest and down to delicate area around his nipple. Katsuki sinks his teeth in a little rougher here, tasting the softness of Izuku’s tit before rolling a petal soft nipple around his tongue. Izuku lets out one of his high, keening noises at that, back arching up of his own volition. It creates a nice space for Katsuki to slip his hand underneath a slim waist, pressing Izuku more firmly against his mouth as he sucks.

Izuku tastes – God, Katsuki would kill for this taste. It’s honey and home and he wants to tear into the kid and make a fucking nest in him. Wants to swallow him whole and make sure no one ever gets to him. Every day with Izuku is another day he has to fight his instincts off tooth and nail. He doesn’t know how long he’s going to be able to resist.

He’s panting as he pulls off, a trail of drool connecting his tongue to the abused nub before he quickly moves to the opposite side. The skin is soft and unmarked and Katsuki can’t help but worry at the other delicate nipple with his teeth, a teasing little bite that has Izuku frantically pulling on his hair.

“Ka – Katsu-nii said no biting.” Izuku says, hands pulling Katsuki up just enough to look at him. It means letting his prize slip out of his mouth, but he likes to let the kid feel like he’s got a bit of power in this situation. If only just enough to pause him.

“Did he?” He says, grin a bit too feral. Izuku sticks out his tongue, and before he can rethink that decision Katsuki’s up and sucking it into his mouth. He hadn’t remembered moving, consciously, but he’s draped over Izuku’s small frame again, pressing his tongue into the boy’s soft pliant mouth and taking.

Izuku, ever the quick learner, just opens his mouth as wide as he can, tongue pressing back in these soft little kitten kicks that drive Katsuki up the fucking wall. He presses his tongue deeper, relearning the taste of him as he sucks at the boy’s mouth. Katsuki draws back only when Izuku gags, throat closing around the tip of Katsuki’s tongue and sending a jolt straight to his dick.

“S-Sorry Katsu-nii.” Izuku’s voice is apologetic and soft, and Katsuki is quick to shush him.

“No, baby, that was perfect. Just perfect.” Katsuki soothes, swooping in to kiss him silent. A few little bird’s pecks have Izuku smiling again, and a couple more scattered on his face have him giggling and pink.

Izuku’s back in good spirits, so Katsuki knows he can start pushing in earnest now. He drags his mouth lower, leaving a trail of kisses wherever his mouth explores. They’re small, delicate things that probably tickle more than anything, but there’s enough intent there that has Izuku’s breath tick up a notch. He pauses at the kid’s belly-button, so small and soft, before delving his tongue in to explore the tiny cave.

He’s already straining at his pants, but he’s not even halfway through his meal, and there’s no way he’s skimping on the appetizer. It’s arguably the best course. Izuku’s practically vibrating off the bed at this point, and a particularly strong jerk dislodges his mouth.

“Stay still.” Katsuki growls, reaching up and pawing at the kid until he finds a hand to grasp in his.

“Ah – hnn, I – I can’t.” Izuku whines as thick fingers thread through his. Katsuki just hooks the waistband of those miniature shorts with his index finger, a low snarl rolling out of him as Izuku’s scent wafts out. It’s so concentrated here Katsuki feels a bit like he’s just been punched in the face. He’s so fucking close it’s all Katsuki can do not to flip him over and rut against him like a teenager. He’s older now, college student. He can control himself.

He will control himself.

He pulls down, just low enough for a soft little cock to spring free. Katsuki licks his lips, stares at the pink handful before pressing a noisy kiss to it. Izuku jerks, and so does his dick. It’s Katsuki’s second favourite part about him, and he’s helpless to stop himself from opening wide and enveloping Izuku whole.

He takes all of him, slurping the hairless balls into his mouth along with Izuku’s cock.

Izuku’s all noises above him, trembling so hard Katsuki thinks he’d shake right onto the floor if Katsuki didn’t have one hand on his stomach and the other connected to his own. He’s so soft and smooth, unmarred by razor burn or stubble, like all boys eventually end up developing once they hit puberty and start shaving. Izuku tastes like sin, like perfection, like sweet milk and cherries and Katsuki has to hold himself back from sucking too hard.

Sensitive. He’s still a kid, be careful.

It’s hard to maintain his thought process when he can feel every inch of Izuku’s little cock begins to harden against his palate. He’s so fucking young, and it takes a bit of coaxing to encourage him up to full hardness, but Katsuki’s nothing if not a hard worker.

Control yourself.

He feels a hand thread in his hair, an impossibly light pressure in comparison to Katsuki’s own strength.

“Kat- su – ah! Kacchan, I can’t.” He whines, and Katsuki’s red eyes dart up to get a better look at him. The kid’s got his own squeezed shut, face completely scarlett as he thrashes around on the bed. God, Katsuki’s only sucking his cock and he’s already so beautifully responsive.

He takes pity on the kid, pulling off with a soft sucking noise and blowing at the trembling cock in front of his face. It’s flushed completely pink, a single pearl of pre-come oozing out and growing in size with every puff of air. Katsuki darts his tongue out to lap it up, inhaling deeply as Izuku moans out gibberish.

He smells it then, Izuku’s most intimate spot, getting wet for him. Katsuki’s dick gives a particularly angry twitch in his pants. He pulls Izuku’s shorts off the rest of the way, leaving the kid in nothing but his stupid gaudy All Might socks. His mouth drops open, scenting the air deeper, letting the delicate scent of young omega saturate his senses.

“Time for the main course.” Katsuki mumbles to himself, before spreading Izuku’s cheeks and ducking down again. This time his head goes lower, lips seeking out Izuku’s private little entrance. He kisses at it first, just small pecks that have Izuku trembling on the bed.

“Katsu-nii it feels all warm. Feels wei – ah!” Izuku starts, crying out when Katsuki grins and presses the flat of his tongue flush against his puckered little hole. He licks at it roughly, broad strokes of his tongue laving at the tiny entrance. Katsuki pushes past the first ring of muscle, fingers digging in to the soft meat of Izuku’s ass as skinny legs wrap around his head. He presses his tongue as deep as he can, thrusting into the kid with sharp jabs before pulling back to suckle at him.

Izuku positively wails for him, and Katsuki barely has the presence of mind to try and listen past it. Listen for noise of anybody else in the house. There’s nothing, just Izuku’s too-loud cries as Katsuki devours him.

He doesn’t know how long he spends down there, long enough to coax Izuku’s small amount of slick into a steady stream. Long enough that the legs around his neck have locked into place. Long enough that he’s drunk on the ecstasy of heat and slick, head swimming at the concentrated pheromones. He drinks it down like his life depends on it, slurping noisily at the hole and trying to get as much of it down his throat as possible.

His mind goes hazy at the taste of him as Katsuki loses time. It’s hard to figure out when he’d gotten two fingers pressed deep, but when he comes to he’s licking around his own fingers, Izuku crying as his body fucks down on the two digits.

“– Nii, Katsu – n –” Izuku breaks off, positively sobbing when Katsuki curls two his fingers and presses against that sweet button that never fails to make Izuku scream for him “Ka – Kacchan, please!” He sobs, hips working to fit the digits deeper inside of him.

Katsuki pulls his head back, one arm grabbing at Izuku’s ankle to get free and hold his legs open. His eyes flicker back and forth between Izuku’s face and the tiny swollen ring of muscle sucking his fingers as deep as they can.

Katsuki knows, that with a bit more prep, this young omega would be able to take him beautifully; would open up for him like a flower in the sun. He feels his knot begin to swell at the very thought of it.

But he can’t.

Instead, he just presses down on Izuku’s stomach more firmly, a third finger sliding in alongside the rest and dragging his mouth further away from the slick mess. It runs down his chin even as he licks the decadent liquid off, there’s so much of it now, it’s everywhere.

He’s so going to have to do laundry before his parents are back.

Little things like that don’t matter though, not when Izuku’s moaning his name like it’s the only word he knows how to say. When he’s leaking tears and wailing like this is their first time all over again. Izuku always makes the best noises for him. Katsuki doesn’t think he’ll ever get tired of hearing them.

There’s no way a kid this young should be able to sound like that, but Katsuki has always had a knack for making the impossible, possible.

“You ready to stop playing now?” Katsuki asks, a shiver running up his spine as Izuku nods frantically. It their cute little code, and the only out for their game that Katsuki has ever offered him. There’s never been a stop halfway option. And he’ll be damned if he doesn’t bring the kid off after getting drunk on his slick.

Izuku’s clearly at his limit, entire body flushed and sweaty with exertion. Katsuki would take a picture, always wants to but gets too caught up every time. Today’s no different, getting to his phone is the last thing on his mind. He watches Izuku’s small cock twitch and jerk, saliva pooling in his mouth before diving in to seal his lips around him again.

Two gentle suckles are all Izuku can take, and Katsuki almost comes in his pants at the first hot burst of cum against the roof of his mouth. It coats his tongue like the richest of creams, sweet and perfect and never enough with how desperate he is to swallow it all down. He sucks harder than he should then, Izuku letting out a garbled mess of words and tugging on his hair hard enough to actually smart.

He knows it’s a tad sadistic when he doesn’t pull back right away – waits for Izuku’s crying to reach a fever pitch before finally easing up and slowing the relentless drive of his fingers.

The kid’s whole body is strung bow-tight even after his cock stops pumping cum into Katsuki’s mouth; only going slack after three thick fingers slide out of him and Katsuki lets his softening cock go with a quiet pop.

Katsuki sticks the fingers in his mouth the moment they’re out, lapping up the slick coating them before ducking in to drag his tongue roughly against the now cherry-red hole. Waste not, want not, and Katsuki is never one turn away from one last taste.

“Ka – ah – Kacchan!” Izuku begs, loose fingers tightening in his hair enough to make sure he doesn’t lose himself again.

He’s so hard he thinks his dick could probably cut diamond, and if he sticks around much longer he can kiss his years of self-control goodbye.

With no small amount of struggle, Katsuki presses one last kiss to the twitching ring of muscle before finally drawing back to observe his handiwork. Izuku looks fucked-out, chest still heaving as he turns his head absently towards the sound of Katsuki’s voice.

“How you feeling, kiddo?” Katsuki asks, running his hands up Izuku’s sides in an attempt to get him to relax. Izuku’s eyes are unfocused, his head lolling back against the pillow as Katsuki tries to help him get his brain back online.

Mmhmm.” Izuku’s barely here right now, and Katsuki cracks a smile at the sight of it, some of his reasoning filtering back in.

“Good.” Katsuki says, clenching his jaw as he climbs off the bed. There’s no way he’ll be able to will this down, but he has to make sure Izuku is situated before he retreats to the washroom to desperately rub one out.

“You gonna sleep on top of the blankets or what?” Katsuki says gruffly, looking at the naked mess in his bed and wishing desperately that he could just crawl back in there and make sure Izuku falls asleep tucked against him.

He snuffs that thought out the moment it catches, shaking it out of his head and bending down to prod at the smaller boy.

“C’mon. You’re gonna catch a cold.” Katsuki grumbles, trying to tug the blanket out from underneath the kid. “Hurry up, I have to go to the bathroom.”

Izuku seems to come back to life at this, an adorable frown creasing his features as he reaches for him. 

“Katsu-nii you always go to the bathroom after we play and I always have to sleep.” Izuku complains, sleepily reaching for him. Katsuki indulges him for a moment, letting tiny arms wrap around his neck as Izuku scents at his jaw.

“I gotta take care of some big-kid stuff.” Katsuki explains, because his pheromones are one thing, but having his come spattered all over the kid is not a smell he can explain away. He cringes just thinking about it, erection wilting at the idea of Auntie Inko getting her son back smelling like sex. Like Katsuki’s sex.

“I’m a big kid.” He complains, whimpering as Katsuki slowly untangles himself and presses Izuku back into the bed.

“Not as big as me.” Katsuki says, finally taking pity on Izuku and leaning down to press one last lingering kiss to the kid’s swollen mouth. “Now be good and nap, I’ll wake you up after I’m done and we can have a bath okay?” He says, flicking at Izuku’s nose gently before smoothing over the sting with his lips.

“And if you’re really good...” He pauses for effect, Izuku looking up at him with those big, hopeful doe-eyes. “Then we’ll watch a movie till it’s time to go home, alright?”

The excited squeal Izuku lets out makes something in Katsuki’s chest squeeze, and he can’t help but steal another peck. Izuku tries to pucker his lips, but he’s smiling too hard, grin practically ear to ear. It’s cute, so cute that Katsuki has to take a deep breath to center himself before be cracks and tries to take this any farther.

“Now, you gonna sleep?” He asks, lifting up a corner of the blanket so Izuku can wiggle his way underneath. He’s nodding, eyes already going half lidded the moment he’s buried under Katsuki’s scent.

“Katsu-nii’s the best.” Izuku says, snuggling into his blanket nest and letting out a long breath. “I love Katsu-nii.” He sighs, thumb moving to his mouth in a bad habit that Katsuki hasn’t rid him of yet.

“Yeah,” Katsuki says, brushing a few stray curls off his forehead and watching as Izuku drifts off. “Love you too Deku.”

Later, when he’s in the bathroom, it’s not hard to pull the vivid image of Izuku cumming on his fingers to the forefront of his brain. If it only take him four strokes while sucking any last trace of Izuku’s slick off his fingers, no one has to know but him.