Chapter Text
He’s Smol
It is a known fact that ProHero Ground Zero (Bakugo Katsuki) and Deku (Midoriya Izuku) are rivals vying for the number one spot on the hero ranking charts. It is not a known fact that the two have been secretly dating for five years, proposed to each other last spring, moved in together, and have quite the active sex life. Not even their parents or close friends are aware of this relationship status and both parties want to keep it that way because what’s worse than having a target on your back? Giving villains alike ammo to exploit a weakness, not that they considered love a weakness, but it had been many heroes’ downfall. Afterall, as much as they enjoyed being the other’s fiancé, they enjoyed ruthlessly competing to achieve set career goals as the Wonder Duo more.
So, imagine 25-year-old Katsuki’s surprise waking up early in the morning to find a child in his bed, not his very grown-up Izuku that had a tongue blessed by the universe and an ass rounded to pure perfection, but a boy no older than perhaps eleven snuggling to his side as if the explosive blonde was a full-blown predator that ordered a child bride off the dark web. He did the most reasonable thing one could do, get as much distance as possible before the police sirens went off and he’d be labeled a registered sex offender.
“WHAT THE FUCK?!” Oh, dear, Katsuki found it difficult to suppress his undeniable urge to yell when his brain couldn’t process information like a proper emotionally intelligent adult.
“Huh!” The boy immediately sat up startled, emerald eyes wide open, green curly hair a fluffy bedhead mess.
Katsuki watched cautiously from the safety of the wall he plastered himself to because goodness gracious this looked very, very bad no matter the context. He was the #3 ProHero, top student of UA when he graduated, natural born genius, handsome as the devil (his fans’ opinions, not his), respected not just in Musutafu but the entirety of Japan, and he currently had an unknown juvenile brat sitting cozy in his room.
Had he kidnapped the boy under the influence of a villain’s quirk?! Was he subconsciously into that sort of thing?! Should he cuff himself, throw away the key, and rot in a prison cell until hell could claim him next?!
“Kacchan?”
He snapped his head up at the mention of his childhood nickname. Ruby eyes focused again on the boy staring at him worriedly, wrapped tightly and cutely around blankets so his face only showed (like a baby swaddled like burrito or sushi). Katsuki felt a tinge of familiarity. Where had he seen that expression before?
“Who told you to call me that? How d’you know my name?” Katsuki’s suspicious tone and narrowed eyes caused the boy to look hurt at the accusation.
“What are you talking about, Kacchan? I’ve known you since we were toddlers.” Blinking in disbelief, Katsuki shook his head since that couldn’t be true. The kid had to have been born ten years ago.
“No way. Nuh-uh. I ain’t buying it. I’ve never met you in my life.” Hearing the conviction in Katsuki’s voice stabbed the boy in the heart.
“How could you say that?!” Angry tears welled in those emerald orbs that planted a seed of guilt in Katsuki’s stomach. “Our mothers are best friends. We grew up together. We went to the same schools.”
“Bullshit, twerp. I don’t got no damn amnesia. Ya delusional? Maybe a psycho fan?” Yeah, that’s a good justification, this kid was a crazy admirer that had the ability to bypass his expensive home security system.
“This isn’t funny Kacchan.” The boy hissed and Katsuki scoffed at his tiny intruder. “I’m not fond of this game you’re playing at, but Kacchan you were my first kiss.”
“First kiss?!” Someone lock Katsuki up pronto, he had desecrated a child’s innocence, tasted the forbidden fruit, cheated on Izuku.
“You were my first boyfriend.” The boy proudly declared, glaring as Katsuki winced painfully.
“Boyfriend!” Did Katsuki have a split personality that had him prey on the young or was he in extreme rejection mode of the probability.
“You were my first time!” The boy hollered in aggravation and Katsuki’s mental state broke.
“What have I done. Those above in the clouds, come smite me where I stand.” The blonde whispered in horror, gripping his hair as he began to pace around the room.
His PR was going to murder him. His friends would slaughter him. His mother would torture him. Izuku, his precious sweet light that warmed his cold bitter heart, would be devastated. He doesn’t drink often, despises drugs with a passion, and assumed he’d kept himself in a constant sober state compared to other ProHeroes and their vices. What went wrong?
While Katsuki internally suffered in silence, the boy threw the covers off him impatiently, and hopped out the bed. That’s when an already awkward to explain situation got even more problematic. Katsuki held his breath, noticing the boy’s clothes were much too big on him. Both the boy’s pajama pants and underwear pooled at his ankles, and the only saving grace Katsuki received to halt witnessing real life kiddie porn was the oversized sleep shirt stopping at mid-thigh like a nightgown.
“Wha…? Why doesn’t this fit me?” Curiously, the boy looked down with furrowed eyebrows, the shirt exposing his shoulder. “Why do I feel shorter?”
Katsuki observed the boy’s growing confusion. Then a gear shifted in his mind. Calming his nerves, he put his thinking cap on. Part of being a hero was to mimic a detective and gather the clues. He swallowed his rational fear, stepping closer to the boy who gawked at his smooth hands in astonishment.
“Something botherin’ you?” Katsuki gruffly asked, trying to play it cool.
“My hands are smaller, and they don’t have scars.” The boy answered, puzzled. Scars? Katsuki wondered if the kid had been abused. Whoo, the public was going to crucify him.
As Katsuki drew near, the boy gasped. The reaction made the man pause, not wanting to frighten the kid.
“You’re way taller than me.” The boy stated, eying the man up and down.
“Duh, I’m way older than you.” Katsuki rolled his eyes at the obvious, the boy shook his head.
“No, I mean of course you’re older by a few months, but Kacchan did you have a growth spurt?” The boy tilted his head to the side, really scrutinizing the blond.
“Puberty and me parted ways years ago.” Katsuki crouched down to eye-level with the boy and licked his lip, shivering in discomfort when the kid hungrily followed his tongue’s movements. Thirsty ass brat. “Say, kid, tell me your name.”
“Midoriya Izuku. You know that though, Kacchan.”
Impossible. Katsuki blinked to clear his non-blurry vision at the boy pouting adorably. Memories flooded into his psyche, trying to figure out if the boy was lying or not. Scanning this wannabe “Izuku’s” face, it all seemed to check out. Katsuki abruptly stood up, grabbed tiny Izuku’s hand, and dragged him to the bathroom. Izuku attempted to protest the manhandling, but his frail body was no match for Katsuki’s toned one. The blonde put the boy in front of a full-body mirror to test his theory. Emerald eyes enlarged, one hand touching his reflection pensively, and the other fingering his chubby cheeks and bird chest.
“Kacchan! My muscles! My undercut! My height! I’m not an adult anymore!” Izuku’s voice cracked, underdeveloped and somewhat endearing to Katsuki’s ears.
“No shit.” Katsuki crossed his arms, amazed that his fiancé was a runt again, and relieved he didn’t commit a heinous crime.
“Oh, no! How?! Why?!” Izuku whined, lifting up the oversized shirt to mourn the decrease in size of his nether regions.
“Hey, Deku! Warn me before you start flashing your underage ass!” Katsuki barely managed to avert his eyes in time, fixing his gaze on the ceiling.
“S-Sorry.” Izuku blushed, letting go of the shirt.
The two went back to looking at Izuku’s reflection. Katsuki’s fiancé was as tall as his belly button. His baby fat had returned too. Izuku held his chin, his face in deep thought.
“I went on patrol yesterday.” That was a start.
“Yeah, you told me to save you some dinner.” Katsuki had remembered their nice phone conversation.
He had been prepping the food as Izuku dressed in his costume at the agency’s lockers. Since Katsuki was a morning person, dayshifts were his go-to schedules, and Izuku preferred the nightshifts. It was common for Izuku to come home a few hours before Katsuki would wake for work.
“Last night I was teamed up with Todoroki. We fought against villains with time quirks and captured them all.” Izuku’s eyes widened in realization. “That’s it! One of the villains, Past Tense, scratched Todoroki and me during battle! That’s probably how I de-aged!”
“Then that means Todoroki is also tiny.” Katsuki added on, not looking forward to that meeting.
“We need to head to the agency.” Izuku said with determination.
Immediately, Katsuki and Izuku power walked to the door. Suddenly, Izuku stopped. He was trapped in his pre-teen body, feeling a breeze between his legs as he wore nothing but a shirt. All the shoes in the apartment were for adults, none for children. Essentially, Midoriya Izuku was practically naked. Katsuki had tied on his own shoes, unaware of the issue.
“Kacchan, wait. I have nothing to wear.” Izuku groaned.
“Fuck. I forgot.” Katsuki peered over at his fiancé, attempting to gauge his size. “Guess, we gotta go shopping.”
“How will I go outside with you?” Yeah, Katsuki carrying a barely dressed boy of whom he had no blood relationship too would be rightfully suspect.
“You just wait here. I’ll grab something real quick. Don’t leave.” Katsuki commanded and rushed out the door.
Izuku pouted, stuck inside until his fiancé came back. That was going to be boring. He decided to distract himself with TV. Surely, the news would have highlights of Todoroki and his fight with the villains. He turned the channels, ready to preserve his maturity, but the bright colors and loud noises of the cartoon channel caught his attention wholeheartedly. Izuku couldn’t understand it, he hadn’t looked at cartoons since All Might’s had its last season, yet his child-sized brain was entranced by the simple plot and goofy characters. Quickly, Izuku forcibly pulled himself away from the glowing screen, running to get his journal and document this stunning revelation.
He was most likely eleven years old.
He still retained his memories, but his body was younger.
Todoroki was most definitely affected by the villains’ quirk like him if he got scratched.
He didn’t know if he retained his quirk while de-aged.
He was hypnotized by the cartoons, so maybe other childish things would get under his skin as well.
His Kacchan needed to hurry up so they can get to the bottom of this because the main question was left unanswered.
How long would Izuku be a kid?