Izuku began taking care of Katsuki when he was 16 years old, their parents knew each other so when Mitsuki and Masaru started traveling a lot for work they decided that Inko’s sweet Alpha Son would be the best bet. Katsuki was to be an alpha with Mitsuki’s strong genetics, her seed could have no less! Having an alpha figure around would do Katsuki good to learn mannerisms and attitude even if Izuku was on the softer side of alphas.
Katsuki was a loud 10 year old, energetic and even somewhat of a bully to his babysitter. He would get on Izuku’s back, hold on for dear life when the alpha tried to clean or cook in the Bakugou home. He would yell at Izuku for trying to pick him up from school rather than their meeting point between the elementary and high school. Izuku would just smile and ruffled his blond hair, Katsuki was almost sure the alpha was messing with him.
Izuku was paid before the Bakugous would leave on their trip, always including extra money in case Katsuki needed to do a project or if food ran out. They left for weeks at a time but spoiled their son horribly when they were home, Katsuki didn’t seem to mind, he video chatted with them, called them often or even texted Masaru to complain that Izuku didn’t make the food like him or that Izuku stayed up until 1 am doing homework.
Katsuki was now 13 in middle school with no sign of presenting anytime soon. He had a little growth spurt, his face wasn’t as round but no scent. Masaru would cuddle his only son near heats and tell him he still smelled like when he was born, soft and gentle. Katsuki let his bearer coddle him because once he presented as an alpha there would be no more of this.
Izuku still cared for him even if he had graduated the year before, he went to college in town so he was on call if Katsuki needed something. He stayed the night when the Bakugous left the city or even the country for work. That night Bakugou insisted that Izuku tried some of his cooking. Izuku held his tongue because eagerly cooking was something omegas did but after the big salty bite he took, maybe Katsuki didn’t have the nature in him to cook.
“If you don’t like it, don't eat it! Idiot Deku!” Katsuki reaches for the plate but Izuku takes it and holds it over his head. The alpha had also hit a huge growth spurt, growing like a tree both in height and width, Masaru's larger clothes didn’t even fit Izuku anymore.
“I’m eating! Just give me a glass of water!”
Izuku forced himself to eat the whole thing, even under Katsuki critical eye.
“Maybe less salt next time? It was delicious, thank you.”
Katsuki glares at him and Izuku notices how the boy puts a hand on his stomach.
“My stomach hurts…”
“From your food?” Izuku gets a little worked, Katsuki almost never got sick.
“I dunno. Whatever, let’s watch the new superhero Movie the hag got me!” Katsuki is quick to make a beeline for the living room and left Izuku to rinse their dishes.
Izuku notices something right away, Katsuki sat next to him for the movie. It was fine but usually, the boy sprawled out on the other couch or on the bean bag chair in the corner of the living room. He put it off as Katsuki feeling sick and without his parents there he must feel lonely. The movie is going well, the sun is setting and it’s getting dark quickly. Izuku crosses his arms over his broad chest, biceps bulging under his tight shirt, his thighs spread as he relaxes into the couch. Katsuki peeks at him from the corner of his eye, he knows what the little warmth in his stomach means, he knows what feeling wet is like.
Katsuki’s heart rate shoots up, thumping in his chest and throat. He brings his knees to his chest, resting his chin on his knees and thinks for a moment. Alphas don’t get wet, Betas don’t get wet. Only omegas… Katsuki was smart, he knew what the signs were. Wanting to be in the proximity of an alpha, cramps, wet slick, fever. He felt flushed but when he peeks at Izuku again the alpha seemed to have drifted off. Katsuki waits another ten minutes once he’s sure Izuku is asleep he scoots a little closer, places a hand on the alphas thick thigh to push himself up. He had to confirm his own suspicion. If he could smell Izuku’s alpha scent then he really was presenting.
His little nose twitched as he buried his face into the teenager's neck, his face turned red as the scent made him release a splash of slick. Everything was so hot…
Katsuki licked his lips and sighed softly, Izuku smelled so nice and he always helped Katsuki out. Was this different?
“K-Kacchan…” Izuku’s large hands push at his small shoulders and they’re staring wide-eyed at each other.
“Deku… I’m... I’m an omega..” Katsuki thinks he’s gonna cry but as he says it, he almost sounds happy, ecstatic because now that he’s an omega. An alpha like Izuku will have to take care of him. “Help? My tummy hurts.”
Izuku swallows hard as the childish language but he loses resolve for a moment, Katsuki climbing onto his lap. Izuku takes a sharp inhale of breath, he can feel the omega slick already wetting his pants.
“Kacchan! We-We have to call your parents!”
“Why! They’re not here! Please… I won’t tell them you’re touching me…”
Izuku’s hand had wandered and grasped handfuls of the boy's soft ass, Katsuki presses his chest against his caretakers and with soft pink lips pressed a kiss on Izuku’s cheek.
“Stop, we cant…” Izuku shoves Katsuki onto the couch and stands up when Katsuki grabs his hand, he doesn’t mean to use his alpha voice but he does. “Stop!”
Katsuki’s eyes well with tears and he sits on the couch, his nose twitching, lips trembling and he’s crying. Izuku immediately loses any anger and gathers the boy in his arms.
“Sorry, I’m sorry. Let’s call your parents and I can call my mom to come to stay with you, she’s a beta so it’ll be easiAH!” Izuku jumped as two hands grab him by the groin, curse his anatomy for being hard around the cute crying omega.
“I want you... To do all the things alphas do, Deku…” Katsuki’s voice is low and Izuku remembers that Mistuki always used alpha voice in the home because Katsuki would not listen most of the time. Izuku groans accidentally, hands are so clumsy and small, feeding his ego to the max.
He can’t take it, he’s losing himself to rut by being around this omega too long. Izuku wants to bite Katsuki’s pretty neck, wants to shove his big alpha cock into the blondes virgin ass, wants to breed him, make him fat with pups, make him cry and scream as he cums. Izuku blinks away is haze and shakes his head. He can’t, it’s not right. The courting age is 15, Katsuki is close but without his parent's consent—Izuku would be in a world of hurt.
Katsuki’s soft wet lips kiss his jaw and under his chin, his face red and flushed, he’s lost in heat now.
“Deku… Please… Here…” he takes Izuku’s left hand, guiding it into the back of his pants, the alphas growling as he’s met with a splatter of slick. He can’t take it, he pushes the omega onto the couch, forcing his pants down dainty legs. Katsuki cute pink cock weeping copious amounts of pre-cum. His pink little hole is twitching and goblets of slick dribble out.
Izuku pushes a finger inside and Katsuki’s voice is so high and needy, a gasping whimper.
Katsuki bucks his hips and slips a hand under his own knee, pulling it up to give the alpha more space, red eyes cloudy with pleasure. Izuku makes quick work, the hole slippery and loose around his fingers, omegas are always eager around alphas they like.
Izuku shoves his jeans down to pull out his cock, it’s huge and heavy in comparison to Katsuki’s. He slaps it down against the omegas, bucking his hips and Katsuki let’s put a shrill cry, throwing his head back.
“Please please please.” Katsuki doesn’t know what he’s begging for but he wants whatever Izuku will give him.
Izuku is only able to push about halfway in, Katsuki trying his best to stay loose but it’s not enough. Izuku growls and pushes Katsuki’s legs up, a leg raising onto the couch and he’s lost too. He’s growling and biting at Katsuki like an animal, the boy moaning and biting back, both covered in bruises of a feral meeting.
Izuku’s cock is meeting the deepest of his parts, it’s in his stomach, making his toes curl, making him sound like the girls in the porn videos he had snuck out to watch with his friends. He’s so thick and long, his alphas scent covering Katsuki. He’s thrusting so hard now, he’s using the omega as a cocksleeve with the intent to knot and cum inside the boy, with the intent to breed the barely presented omega.
Izuku wraps an arm around Katsuki’s back, forcing him to arch and he digs his teeth into the boy's neck, the taste of Pheromones and blood filling his senses. He pushes in his fat knot even against the protests of the boy, even as he’s slapped and as hands push as his chest. He’s cumming so much he’s not sure his knot will keep all of his cum inside, he keeps grinding into Katsuki. By the way his walls twitch and then tighten, the way a new smell enters the room he knows Katsuki came by instinct. Omegas always had a higher chance of pregnancy if they orgasm while knotted.
Izuku takes the time while knotted to lick at Katsuki’s tears and red cheeks, to lick at all the bites and at their bond mark, lapping up the last of the blood. He wants to go again and again already, he wants to have the boy stretched out and loose to fuck at any moment.
It’s like that for three days, moving from the living room to Katsuki’s bedroom. Izuku in alpha rage ripping bedsheets and breaking the bed to have it exactly
How his omega wanted it. Katsuki rewarding him by some inexperienced riding, his pale thighs trembling, his chest heaving, his pink dick coming over himself as he begged Izuku for his fat knot Over and over.
Katsuki purring as his alpha enveloped his body with all that muscle, nice and warm for them to sleep
Izuku blinked and immediately covered Katsuki’s body as the scent of alphas drew closer to their room. Mitsuki slammed open the door and stomped over, grabbing Izuku by the hair and wrist to throw him off the makeshift nest as if the teenager wasn’t naked. He’s growling but under the guise of an older, fatherly alpha he had to submit, Katsuki’s big worried eyes making him submit. He could never fight his omegas sire.
Izuku isn’t allowed around anymore, Mitsuki says not until Katsuki turns fifteen to at least be at a suitable age to court. That doesn’t stop Izuku from walking Katsuki home through back alleys, getting a quick fuck in public restrooms, he’s angry that Mitsuki put Katsuki on birth control.
He’s not worried, the second Katsuki is 15, he’ll legally become the boys alpha. He’ll take the omega off the birth control and force him to have all of his pups. Katsuki looks blissed out the more Izuku talks about it, just calls his alpha an idiot before kissing him and running home so that his parents don’t get too suspicious.
Izuku didn’t realize how possessive he was but when Katsuki turns 15 no one will stand in his way.