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The first time it happens, it’s an accident. Totally innocent.
They’re on their way to the agency daycare centre, Kazuko’s tiny fingers curled around their hardy, calloused palms as she swings between them. She stops suddenly, her little face mimicking her father’s perpetual scowl as she kicks out a foot and looks up expectantly, laces dangling.
“Papa! My shoe!”
The look of absolute malice is cute on a three-year-old.
Izuku squeezes her hand, affection surging through his chest as she glares back up at him.
“Don’t worry, baby, Daddy will fix it for you.” He turns to Katsuki, giving him a sly wink. “Won’t you, Daddy?”
The red flush that spreads up Katsuki’s throat is unmistakeable, as is the sudden swirl of hot, dark scent in the air to match. The alpha drops to his daughter’s feet with a choked off splutter.
“Don’t fuckin’ call me that,” he grunts, flicking Izuku a glare to match Kazuko’s as he swiftly ties her laces.
“Language,” Izuku mouths, jerking his head toward the blonde-haired toddler between them. “Call you what?”
Katsuki pointedly averts his gaze, standing and taking Kazuko’s outstretched hand, tugging her along.
“Doesn’t matter.”
Izuku trails behind, watching as Katsuki stomps off with their daughter in tow.
“No, come on, don’t call you what?” He protests. “Daddy?”
Katsuki trips on the sidewalk.
“What the fu – I said it doesn’t matter, okay?”
Izuku takes stock of his mate: red-faced, agitated, disagreeable. The tell-tale signs of Katsuki’s discomfort that Izuku can recognise at the drop of a hat after all these years. He’s embarrassed.
Interesting.
Raising a smug brow, Izuku leans in to bump Katsuki’s shoulder with his own when he catches up. He bends to sweep Kazuko up into his arms, the familiar heavy weight of her chubby limbs and the stickiness of her palms ever a comfort.
“Okay, Kacchan, whatever you say. Let’s get this little hero to daycare.”
***
Later, Izuku is leaning against the door of his office, a nonchalant hip pressed up against the frame as Katsuki prepares to head out on patrol. He watches as the alpha cracks his knuckles, fully decked out in hero gear – bruising for a fight and menacing as ever.
When Dynamight barges past, Izuku grabs his opportunity, stepping in close and blocking his path.
“Whaddya want, Deku? I’m about to head out.”
Izuku only smiles and ducks his head under Katsuki’s chin, circling his arms around Katsuki’s nipped waist. The alpha promptly goes stiff as a board, narrowing his sharp eyes. It’s rare for Izuku to display even a hint of omega subservience. The push and pull is what they thrive on.
“What’s up with you?” Katsuki asks, suspicious.
“Can’t I say goodbye to my mate once in a while?” Izuku pouts, Kacchan’s stubble prickling his cheek as he leans forward to whisper into his ear. His voice curls out like the drip of honey, intentionally seductive. “Go get ‘em, Daddy.”
And with that, Katsuki’s scent spills out into the corridor like a distress flare. If a distress flare could be horny.
“You little – you little shit!” He gapes, slightly breathless before the anger kicks in.
Izuku grins wildly, reaching up to ruffle Katsuki’s hair and kiss his cheek before hurrying back to the safety of his office.
“Good luck out there, Dynamight!” He calls cheerily, shutting the door behind him.
He only flinches slightly when the roar from the corridor rattles the door on its hinges.
“You an’ me are gonna have a little fuckin’ conversation about this later!” Katsuki rages, Kirishima’s placating tones just about audible over the din as Katsuki’s threats grow progressively distant.
“Oh, I hope so,” Izuku says aloud.
***
When Katsuki arrives home that night, it's with irritation simmering beneath his skin.
“Where’s the brat?” He asks, kicking off his boots in the entryway, face darker than the deepest hour of the night.
“At my mom’s,” Izuku replies evenly, not looking up from his spot on the couch, flicking over another page of Ultra Unlimited magazine with all the indifference he can muster. “I asked her to babysit.”
He doesn’t avert his eyes from the page, not even when two heavy feet land solidly in front of him.
Katsuki’s scent coils through the air, hot and heavy, the heady mix of musk and wood-smoke stifling Izuku’s lungs.
“The fuck is your game, freckles?”
Izuku forces himself not to look up, to stay intently focused even as slick begins to pool between his legs in response to his alpha’s voice.
“Hmm?” He says, casually raising a brow. “What do you mean, Kacchan?”
“You know exactly what I’m talking about, fuckface, don’t act dumb.”
Izuku looks up at last, eyes flitting over Katsuki’s brows, knitted together tight and intense, lips curled into a snarl. Fuck. He wants Kacchan to destroy him.
Swivelling his knees languidly to one side, he lets his feet drop from the couch and onto the floor. Danger pulses through the air on a wave of Katsuki’s scent, and Izuku drinks it in as he stands.
“You mean you don’t like it?” He asks sweetly, curling his fingers around Katsuki’s neck as he brings their faces together. “Daddy?”
The groan that tears between them is that of a tortured man, Katsuki’s eyes clenching shut as he sinks into Izuku’s touch.
“Fuck. You.” He croaks, breath a hot rasp over Izuku’s lips.
“Mm, yes please,” Izuku murmurs, before yanking him in for a kiss.
The moment their mouths meet the world starts spinning, spiralling on its axis like it’s about to careen off into some distant galaxy. Their fingers are quick and their patience burns through in seconds.
“On – the – floor,” Katsuki instructs between lunges of tongue, hands gripping Izuku’s hips hard and hungry. “Get on the fucking floor.”
Izuku hears himself whimper as he obeys, dropping to his knees as Katsuki wrenches his shirt over his head. Clothes go flying across the room, torn from charged skin without care or attention.
Nothing about their movements is gentle as they strip each other down, Katsuki wrenching Izuku’s face up for a last ravaging kiss before pushing him down onto hands and knees.
The floorboards are cold and hard beneath him, but Izuku’s head spins dizzy with the burnt spice of his alpha’s scent and everything else is forgotten.
“You wanna mess with me, huh?” Katsuki growls, a dribble of slick sliding down Izuku’s thigh. “Let’s see how brave your smartass mouth is when you’re bouncing on the end of my dick.”
There’s a heavy slap between Izuku’s bare, presented asscheeks when Katsuki lifts his cock and rests it heavily between them, rutting slowly from behind, coating himself in slick.
“Mm, Kacchan, I – I’ll be good,” Izuku gasps, rocking himself back with a needy whine. “Just – just fuck me.”
“Oh, I’ll fuck you, omega,” is Katsuki’s responding rumble. “I’ll fuck you when you ask for it right.”
“F – fuck me, alpha, please!”
Katsuki chuckles, low and dangerous.
“Nope. Try again.”
The only sound beyond Izuku’s harsh intake of breath is the slip of Katsuki’s dick against wet skin.
“Fuck me… fuck me, Daddy.”
The world goes resoundingly white, the quick stretch of Izuku’s sopping hole so sudden that he jolts forward and knocks his brow against the floor. Katsuki pauses, hilt-deep, flattening his palms against the small of Izuku’s back.
“Fuck yeah, that’s right.”
Then his hips stutter forward like a loaded gun, filling Izuku deep, again and again. Izuku’s fingernails scrabble against the floor, desperate to find something to hold onto as his alpha fucks into him, each thrust hard and unforgiving.
“Oh yeah, like that, Daddy,” Izuku moans, revelling in the wild animal that’s been unleashed upon him. “Just like that.”
The cord strung up between them tightens with every jerk of Katsuki’s hips, every pulse of Izuku’s twitching hole as he moans and gets lost in the pleasure. His cheek pressed to the floor, he’s ready to pass out with overstimulation when the tight grip of fingers wrap around his cock.
All it takes is a few deft strokes and Izuku is staring into the abyss.
“Kacch – Daddy, I’m gonna –“
“Fuck.”
The hot, seething end of Katsuki's desire fills Izuku deep – his big, alpha frame shaking and quivering against Izuku with the force of it. Izuku's own toe-curling orgasm is quick to follow; long and drawn out, despite the quick fuck, his arms collapsing out from under him as he writhes into Kacchan’s hand.
“Oh, holy fuck.”
The cool floor is the only reprieve from the intense heat that surges through Izuku’s every nerve ending.
When it’s finally over, Izuku stirs in the wake of their desperation, grimacing as he notes their dirtied mess of a floor, covered in hastily discarded clothing and bodily fluids. With Katsuki still inside him, the alpha’s breaths ragged and harsh against his back, Izuku can’t help but let out a laugh that wracks him from head to toe.
“The fuck, nerd? You think this is fucking funny?”
Katsuki’s voice is raw, cracked. Maybe even a little ashamed.
Izuku disconnects their bodies, ignoring the stream of slick and spend that cascades to the floor as he rolls onto his back and pulls Kacchan into a tight embrace.
“No,” he says, wrapping his arms around his alpha’s neck, stroking into his hair. “I think it’s fucking hot.”
***
Yep, the first time Izuku calls Katsuki Daddy, it’s an accident. But every time after, it’s definitely on purpose.