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Things get much easier after their conversation, to the surprise of absolutely nobody. Turns out because they were practically joined at the hip anyway, very few things in their dynamic changed; the only new additions were the good morning and good night kisses, the I-love-yous, and, well.
The box.
It took a while for them to work up to it all, because Katsuki believes in doing things properly, and he wasn’t gonna rush into all this kinky shit with Izuku and then accidentally fuck both of them up. They’d had—scheduled, actually, at Izuku’s insistence—a proper sit-down conversation about both of their likes and dislikes when they weren’t both frazzled and horny out of their minds. Izuku likes being praised, he likes being spanked, he likes his hair pulled, he likes being used. Izuku does not like being ignored or degraded. Doesn’t want to feel like he’s upsetting Katsuki. Which is all good. Katsuki just wants to be in charge.
It starts with the collar. If the leash is on it too, that’s for sex. If there’s no leash, he just wants to unwind. And it does something to Katsuki, sexually, emotionally, knowing that branding KACCHAN on his neck calms Izuku. Grounds him. He grounds him.
After a month or so comes the muzzle. Izuku had placed it on top of the box shyly at first, but fucked Katsuki so good he ate Izuku out and gave him another two orgasms as an extra award. Katsuki loves when Izuku wears the muzzle. He gets so desperate and whiny, like the dog he is.
It took a while for them to work up to the dildo, not for any real reason—well, they were just so pent-up for so long they kept cumming early, but that’s neither here nor there. Once Katsuki had made Izuku wait to fuck Katsuki all weekend, only for him to cum as soon as Katsuki’s knuckles brushed against Izuku’s tdick. Katsuki had crowed about it for days until Izuku discovered what prostate milking was.
No more jokes were made about premature ejaculation after that.
The first time Izuku fucked Katsuki with the dildo, they both cried all over each other. It felt so good, Izuku filling him up, stroking his cock. It felt so right. They took a lot of pictures of Izuku in the harness that day, too. It filled Katsuki up beautifully, but it fit Izuku far more; the way it seemed to curve slightly to the left, the size, the dark green that matched the thicket of hair between his legs. Izuku was complete with or without the silicone, both of them knew, but something about his glow made Katsuki feel more whole. He started crying before Izuku did, and then they were both just a mess.
But these days are Katsuki’s favorite. The weekends where Izuku doesn’t have heavy grading and neither of them have shifts. Weekends where Katsuki can be as rough or as gentle as he pleases, and all Izuku can do is say “yes please,” “more please,” “yellow,” or “red.”
It begins after lunch on Saturday. Izuku starts to get fidgety and Katsuki knows it’s time because Izuku is never fidgety, not during All Might: A Golden Age. It’s his fourth favorite All Might documentary. His hands get twitchy, he keeps shaking his leg, so Katsuki takes pity on his boy.
He turns the tv off and looks at his boyfriend. “You wanna be Deku?”
“Yes,” Izuku breathes. “Yes please, Kacchan.”
Katsuki rewards him with a short kiss. “Good boy. Get the box for me, and then we’ll get ready.”
Izuku already looks more settled when he comes back into the living room and presents the box to Katsuki. He places it on the coffee table, exactly where it was the first time, and sits eagerly on the floor between Katsuki’s legs.
“You look good there,” Katsuki lets himself say.
“Feels good here.”
“Hm.” Katsuki takes the collar out of the box, fastens it around Izuku’s neck, checks the give. “You gonna be a loud puppy tonight, baby? Do you need the muzzle?” Izuku shakes his head. “Alright, baby, I’m trusting you to behave, okay? You gonna do that?”
“Yes, Kacchan.”
“Good boy,” Katsuki says, clipping the leash on. “You can put your harness on now.”
When Izuku is strapped in, Katsuki lets himself run his hands down Izuku’s torso, feeling the fine hair that runs from his belly button to the top of the strap. He scratches gently, and Izuku’s hips jerk.
“Uh-uh, Deku. C’mon, to the room before you make a mess.”
Katsuki jerks the leash a little, and Izuku pads behind him into the bedroom, standing eagerly at the end of the bed while Katsuki lays down. His eyes are hungry and desperate, just how Katsuki likes it.
He’ll be nice. Izuku had a long week, and Katsuki’s getting impatient. His legs spread and Izuku zeroes into the spot where his legs meet.
“Can I make you feel good, Kacchan?”
Katsuki cocks his head to the side. “Can you ask politely first? Don’t be dumb, Deku.” He tucks on the leash, making Izuku lurch forward on the bed. He immediately takes advantage of it, crawling over Katsuki until he begins to pepper his neck and collarbones with light nips and kisses. Katsuki can’t help but break character for a second to smile up at the ceiling, helplessly endeared by his boyfriend’s never-ending affection.
Izuku pauses, resting his cheek on one of his boyfriend’s pecs, to look up at him. Puppy eyes, Katsuki grouses. Stupid fucking puppy eyes. Katsuki shuts his eyes to take a deep, slow breath, the force of it making Izuku rise and fall along with his chest. He will not succumb to the cuteness aggression and do something stupid, like pinch Izuku’s cheeks and pull them. Or kiss his dumb dumb stupid dumb pouty lips and hug him tight to his chest and wait until they fuse into one person. Or something. Whatever.
Izuku waits patiently for Katsuki to collect himself like the well-behaved mutt he is.
“Sir,” he says, “May I please make you feel good?”
Something in Katsuki clicks into place when Izuku calls him sir, something twisted and possessive and right, and he lets himself sink back into the heady headspace of control.
Katsuki reaches around Izuku’s neck and yanks, pulling his dog up by the leash. The strained oof! And cough Izuku lets out from his airways being blocked just makes Katsuki harder. He doesn’t let go of the leash.
“Of course, Deku. What else is my puppy gonna do?”
When Izuku smiles dopily, eyes glazing over with submission once again, Katsuki releasees the leash so Izuku can get between Katsuki’s legs, shucking off his Master’s pants.
“Fu—” Deku starts, seeing Katsuki isn’t wearing any underwear, before remembering the rules for today. No talking. So he swallows and whines, licking his lips desperately as he lowers his mouth to Katsuki’s length.
Just seeing him was enough to make Katsuki fill out a little, but Deku swallows him in one go, swirling his tongue expertly around the head, humming when Katsuki’s hips buck up.
“Fuck! Ah—Deku, yes—like that, good boy—”
Deku squirms with the praise, nosing along the side of Katsuki’s cock while pressing his thumb insistently on Katsuki’s perineum. With each press, Katsuki lets out more choked, cut-off moans, yanking the leash closer and closer to him. And with each press, Deku is choked around his collar, over and over again. His slick wets through his dark boy shorts, dripping down the insides of his thighs, but Deku doesn’t notice and doesn’t care, too focused on caring for his Master.
Deku whines again when he notices his Master’s abs start fluttering under his attention, releasing Katsuki’s length with a lewd slurp. He lets his hands knead the inside of Katsuki’s thighs, kissing messily along the fine hair there, leaving a trail of bruises and spit. And when his thumb reaches Katsuki’s rim, already a little loose, Deku moans loud and low. When he looks up to his Master, shocked, he finds Katsuki propped up on his elbows, looking down at him with a smug little smile, eyes dark with lust.
“Yeah, I opened myself up for you, you greedy mutt. Are you gonna finish the job? You gonna thank your master?”
There is a bottle of lube already within arm’s reach, but Deku is so overcome with lust he bowls Katsuki back onto his shoulders, knobby hands gripping the meat of his thighs so tight they’ll bruise, to expose Katsuki’s hole.
When Deku props his Master’s legs over his shoulders, takes Katsuki’s length in one hand, and spits right into his hole before diving desperately in with his tongue, Deku nearly cums, totally untouched, at the way Katsuki sobs, hiccupping and shaking around his shoulders with lust. Yanking the leash once more, Deku lurches forward and Katsuki reaches up to grip his biceps.
“Good—fucking—puppy,” Katsuki grits out, a cut-off whimper echoing through his skull as the praise makes Deku slobber around his Master’s rim even more desperately. “So—ahnn—good for your Master, huh? Only good for—mm, yes, right there Deku—one thing, ain’tcha? Huh? Say something, ya dumb mutt.”
When Deku looks up from between Katsuki’s legs and squeezes Katsuki’s length in his fist even tighter, he looks practically feral. His bangs are plastered to his forehead, mouth swollen, brows furrowed over eyes swallowed with black, a flush that drowns his freckles seeping to his chest. From this angle, Katsuki can already see the beginnings of the red, dug-in skin around Deku’s neck, the seams of the collar.
When the scene is over, after Deku is orgasmed-out and cuddled and warmed-up and bathed, after he becomes dopey, stupidly clever Izuku again, Katsuki knows they will both trace over the sensitive, raised flesh he can’t see right now, knows he will probably kiss it, bite around it in the shower to leave the imprints of his teeth there, that both of them will probably get heady with arousal while cleaning up, that Izuku will look in the mirror and let overwhelmed, blissful tears drip down his cheeks while he looks at it.
Knows that the love of his life, his hero partner, his boyfriend, his dog, is about to fuck him. And that the whole time he does, “KACCHAN’S” is getting pressed deeper and deeper into his neck, branding him.
Shit, Katsuki’s about to cum at the thought.
When Katsuki zeroes back in, everything is too sensitive, almost painful. His legs are still over Deku’s shoulders, and Deku has four fingers in Katsuki’s ass, his other hand gripping the pillow near Katsuki’s head, and his mouth is open, drooling freely on Katsuki’s chest, panting desperately.
And now that Katsuki’s done fantasizing about what’s going to happen about a half-hour from now, he realizes he’s about to cum because his dog of a boyfriend is nailing his prostate with every thrust of his fingers, using Katsuki’s precum to wet his length as he drags his fist back and forth over it.
He feels his abs clench again, and that is simply unacceptable. The only place Katsuki is cumming tonight is on his dog’s cock.
Katsuki yanks on the leash again. “Alright, alright—shit—you dirty mutt. Move.”
Deku perks up excitedly, head jerking up so fast that the ears nesting in his hair jostle, making Katsuki snort. With room to move now, he flips onto his stomach, keeping the leash over his shoulder. When he looks back, Deku swallows audibly.
This is the best part, Katsuki thinks—knows, really. When Deku has finger- and tongue-fucked him to near-oversensitivity, so desperate for his Master’s release that he gets messy and desperate, needing to be inside his Master so bad that he abandons all technique and just pounds into Katsuki’s hole, drooling and sobbing along his back. Katsuki smirks toothily, looking over his shoulder and narrowing his eyes.
“C’mon, Deku,” Katsuki says, jostling the leash a little. “Come fuck your Master.”
At the end of the day, Deku is the best dog a Master could have, desperate and obedient and so eager to please. So he spreads his Master’s cheeks with one hand and lines himself up with another, then presses in all at once.
Shit, there’s nothing in the world that can compare to being filled up by Deku. Something carnal, but something right. And without fail, Katsuki knows that every time he looks over his shoulder, or up above himself, or into the mirror, he will see Izuku looking at where they meet, knuckles white as his fingers squeeze giddily around Katsuki’s small waist, joy plain across his face though tears fall down to his cheeks. And every time, whatever they’re doing, Katsuki will smile, too, close-lipped and genuine.
Then the initial stretch of Deku’s strap turns into burning hot pleasure, and Katsuki forces Izuku forward by the neck and snarls “Fuck me, pup. And be a good boy about it.”
Deku gets to work immediately, knowing his Master doesn’t like any of that ramp-up bullshit and presses Katsuki back into the mattress, thrusting hard and deep, brushing against Katsuki’s prostate with every push of his hips.
When Katsuki feels wetness splash against his back, he knows that it’s either his puppy’s drool or tears, and that both of them are probably gonna cum soon.
“Is my puppy feeling good? Does Deku want to cum for his Master?”
Deku breaks a rule, but Katsuki is so on edge, cock rubbing against the sheets with every jostle of his hips, he can’t find it in himself to give out a punishment. Especially when he sounds so cute.
“Yes—hah--- yes, Master, Kacch—hah, ah! Please, please. Please let me make you cum, let me make you—nngh—”
“Fucking—do it, good boy, c’mon, c’mon Deku, good dog, make Master cum on your cock, you can do it—”
Katsuki only barely manages to get that sentence out without whining like a bitch in heat with the way Deku has started thrusting harder, losing his rhythm, his forehead grinding into the valley between Katsuki’s shoulder blades as he pounds his Master into oblivion. And Katsuki begins to feel it, the tightening and fluttering in his abdomen, the knot in his gut beginning to tighten, the tears beginning to gather in his eyes.
“Deku—” Katsuki pants. “I’m—close. Close, puppy, ah, hah. I’m close, C’mon. Fuck Master, fuck me, FUCK ME—”
Katsuki cums, spilling over onto the sheets and keening high-pitched and airy, toes curling while his legs go ramrod-straight between Deku’s. Like a good dog, Deku keeps thrusting, carrying his Master through his orgasm, until he finally slows to a stop and laves Katsuki’s back and neck with wet kisses.
After a few deep breaths and releasing the leash for the first time since the scene began, Katsuki hums low in his throat.
“You can pull out now, puppy,” he says, swallowing around the gravel in his throat and wincing with the movement at his abused hole. He turns onto his back and lets his eyes run over his lover, taking in Deku’s heaving chest, the blush that travels down his torso and makes the scars under his pectorals seem even brighter, the sweat dripping down his body, the sheen around his thighs.
Katsuki’s cock twitches.
“Did so good, Deku, that’s the way,” he smiles, body lax. “You fucked master very well.” Deku just looks at his Master patiently, on his knees, waiting for his next task. Such a good dog, Katsuki thinks. Trained my baby so well. “Does my good dog want a treat?”
Deku nods so vigorously it startles a laugh out of Katsuki. “Alright, alright, you eager little mutt, don’t give yourself a headache. Let’s get you out of that strap and onto my face, hm?”
And oh, as much as Katsuki knows Izuku loves putting that strap on, he knows Izuku loves the moment he can take the strap off, because it means his Master is going to give him a reward. And Master is going to let him cum.
Now completely bare, Deku looks back at Katsuki again, who moves his elbows and lays down fully on the bed. Katsuki finds the end of Deku’s leash again and grips it tight.
“You ready to ride Master’s face?” He yanks on the line.
“Yes, please, Master,” Deku says, thighs bracketing Katsuki’s shoulders.
“Jesus Christ, you’re soaked. All this for me, puppy?”
Deku doesn’t respond, just whines and kneads his hands through Katsuki’s hair, gripping and pulling. He cocks his head, and Katsuki knows the question he’s asking. Ready?
Katsuki doesn’t say anything either, just nods and brings Deku’s cunt to his mouth.
With a mouthful of Deku’s core, Katsuki’s first thought is mmmmfgh. His second thought is, damn, did his dick get bigger? Could he fuck me with it?
Katsuki’s third thought is ohhhhhhmyfuckinggodmmmmmfgh.
His tongue swipes through the wetness of Deku’s slit before closing his mouth around Deku’s tdick and sucking hard. By his ears, Katsuki can feel thighs beginning to squeeze, trembling with self-control, trying to not to crush Katsuki’s skull.
He remembers the man on top of him has deadlifted a bus with only five percent of his quirk, and his spent cock tries valiantly to fill out again.
Katsuki suckles on the head of Deku’s tdick, practically making out with it, reveling in the frantic groaning and panting above him. He takes the hand holding the leash and almost punches with it, so Deku is jerked back by the neck and forced to push his cunt further into Katsuki’s mouth and brace himself on Katsuki’s legs.
“Use your hips, puppy,” Katsuki groans into Izuku’s hole. “Fuck my mouth, Deku.”
All Deku needed was to be told, it seems, because he immediately starts rolling his hips against the flat of Katsuki’s tongue, sobbing in earnest every time Katsuki rolled his tongue up to flick at Deku’s tdick at the peak of every thrust, or when he found a way to spear his tongue in Deku’s hole, tounging at the sensitive skin around the entrance.
Katsuki’s entire face is soaked with Deku’s slick, and when he takes his free hand to wriggle in three fingers and crook them inside, he just opens his mouth happily, drinking the rush of slick like syrup.
Deku’s sobbing has grown louder now, and his whole body is writhing with the force of his pleasure, thrusts getting uneven. He went from broken “aaahn—aah! Anghhhh, hmm” to going completely soundless, sobbing and panting and wheezing in earnest. There’s no warning, but Katsuki knows his Deku inside and out, and can tell when the trembling of the thighs around his ears begins to ripple to right beneath his belly button, he’s getting close.
He yanks the leash again, over and over, smiling into Deku’s cunt with the garbled sobs and whines he lets out. His thrusts are faster now, not even fully there, just rubbing insistently up against the rough texture of Katsuki’s tongue as Deku twitches uncontrollably. Katsuki keeps his fingers pressed to Deku’s g-spot, keeping an insistent pressure on it, releasing only to curl his fingers further inwards before returning, until Deku begins to wail.
Liquid rushes around his fingers and onto his face and hair as Deku falls backward, losing his grip on Katsuki’s legs as his thighs shake with exertion. Katsuki just keeps his mouth open, lapping up Izuku’s cum, drawing out as much as he possibly can with his fingers, almost ignorant of Deku’s sobbing as he lies backward on Katsuki’s body. With every spurt of juice, Katsuki laps and swallows, laps again, sucks, like he’s the dog now, desperate for his thirst to be quenched.
Eventually Katsuki swallows for the last time and Izuku’s body quiets, both of them heaving—Katsuki from the soreness in his jaw, Izuku from the force of the orgasm he just had. Wordlessly, Izuku rights himself, laying chest-to-chest with Katsuki, letting their breathing sync.
But before they take a nap, Katsuki has one more thing to do.
“Izuku,” he murmurs into his lover’s ear. “Baby.”
“Mm.”
“You Izuku now? Or still wanna be Deku?”
Katsuki feels a shrug against his shoulders. “Mm-hmm.”
“Just one word, baby, and then you can be quiet again, okay? Izuku or Deku?”
Izuku exhales, long and low, against the column of Katsuki’s throat. “Iz’ku,” he says. Swallows.
“Okay, sweetheart. Can I take these off you then?” He reaches up to thread into the ears clipped into Izuku’s hair, runs a hand over the brass buckle and O-ring at the back of the collar.
“Uh-huh.”
“Okay, love. Good boy, thank you for answering,” Katsuki whispers. He unclips each ear and tosses them to the side, then slowly unbuckles the collar, shushing Izuku gently when he begins to whine.
The skin there is red and irritated, and Katsuki will put lotion on it after their bath, but he kisses it now, bites it affectionately, loving the happy little squirm he gets from the dense mass of muscle on top of him and the way his incisors look as they frame his nickname.
“I’m gonna make us get up in a bit,” Katsuki grumbles half-heartedly.
“Mmkay,” Izuku replies. “Nap now?”
“Yeah,” Katsuki half-laughs. “Nap now.”