The first time Izuku breaks her arm, she is three.
Izuku and Katsuki are walking hand in hand down the street, giggling and tired from a day of playing at the park. Mama and Auntie are walking behind them, talking about Grown Up stuff that Izuku and Katsuki don't really understand and don't really care about. Katsuki is putting a foot in front of hers, but she steps over it with a laugh. It's one of their favorite games—trying to see which one can trip the other.
Katsuki is turning four soon; he tells her about the birthday party he's going to have, and how all the kids from preschool are invited. As his birthday draws nearer, his hands produce little puffs of black smoke.
"I'm going to have a cool quirk!" he brags, excited. He cheers when he manages to make Izuku stumble, even though their fingers are entwined, and she almost brings him down with her.
"Maybe we'll both have fire quirks?"
The black smoke reminds her sometimes of her daddy's quirk; he always coughs thick plumes of smoke after he breathes fire. She doesn't know if her quirk will be like her mom's or her dad's, but she doesn't really care. She just wants to grow strong and be a hero – like All Might!
They reach the corner and wait, patiently, for their mothers to come so they can cross the street. Mama and Auntie are still further behind them, their discussion leaving Mama looking teary eyed and Auntie furious.
Katsuki opens his mouth to answer, but before he can, Mama and Auntie are screaming, loud, piercing wails that cut the air and make them flinch.
Izuku's hand immediately clamps onto Katsuki's when she sees the reason Mama is screaming. A car comes hurling from across the intersection straight towards them. There's a second of indecision where she and Katsuki just stare at it before Izuku shrieks, reflexively throwing her arm out in a futile attempt to protect herself.
There's a loud crash, the sound of crunching metal and shattering glass, and then Izuku's arm burns. It aches and throbs and it hurts so bad she screams and screams and screams.
Mama is by her side not long after, crying and checking Izuku over. Izuku's arm is already purple, bent and misshapen. She can barely hear Katsuki crying over the sound of her own wails, and when she looks over to him, he is bleeding profusely from a cut just above his left eyebrow, which makes her cry even harder.
Those are their only injuries, a broken arm and a relatively small laceration.
Someone has already called for the police and an ambulance, and all first responders stare in awe at the bent front end of the car that should have, by all means, killed the two three-year-olds. It looks as if it crashed into a metal pole, like an unstoppable force met an immovable object, and took most of the blow, pushed back some ten feet.
Mama holds Izuku in her lap, trembling yet firm, and it helps calm Izuku down enough so that the paramedics can give her a look over. Her broken arm is the only injury she has, and it is quickly splinted before she is loaded into the back of the ambulance.
"I don't understand," Mama says, clutching her purse in her lap. "How is this possible?"
Izuku is sitting next to her, sucking on a lollipop the nice doctor gave her. Her left arm is in a cast that she and Katsuki decorated yesterday. The cast is now a colorful array of reds and oranges, and even some greens. There are little doodles all over it, but Izuku's favorite is the one Katsuki drew of them as super heroes.
Cartoon-Katsuki has fire coming out of his hands. His quirk, explosion, had emerged early, too. He'd delighted in showing Izuku, making small blasts right behind her head and scaring her into some bushes. Cartoon-Katsuki has grenades for hands, Izuku's own design, and sloppily written underneath is 'King Explosion Murder'.
Cartoon-Izuku is shorter than Cartoon-Katsuki, something that had made her pout and Katsuki laugh not unkindly at her. Her costume makes her look like a giant, green rabbit, and the arms are disproportionately large with bulging biceps. Katsuki scribbled 'Deku' underneath. He'd meant to write 'Izuku' but he's not terribly proficient with kanji yet, and he stubbornly refused to believe that he wrote it incorrectly.
Izuku doesn't mind, though, because Cartoon-Katsuki and Cartoon-Izuku are partners, and they just stopped a villain from robbing a bank. They're a hero duo, equals, and it's all that Izuku wants when she grows up.
"It doesn't happen as often these days, but there is still always a fifteen percent chance of a child developing a quirk dissimilar to their parents," the doctor says. He offers Mama a small, but warm smile. "And while it is uncommon, it is not rare for a child to come into their quirk before they turn four. Given the circumstances, I'd say her quirk activated to save her life."
Mama's hands tremble when she pats Izuku's head, but she smiles softly when Izuku looks at her. "I see," she says quietly. "Her quirk…why did it break her arm? Is it going to do that every time she uses it?"
The doctor frowns a little. "From the tests we ran, we were able to determine that Izuku's quirk is an enhancement one. When she saw the car coming toward her, her hormones spiked with large amounts of epinephrine and norepinephrine, which is the body's natural response to stressful situations. Now, her quirk is a rather strong one, but quirks are physical abilities. Simply put, her body is too small and too underpowered to handle her quirk right now."
Mama nods her head slowly. "I'm afraid I don't really know to…to go about…what I mean is how do I change that? How do I ensure that she grows enough to handle her quirk?"
The conversation continues for a long time, but it's more Grown Up stuff and something about specified diet plans, bulking, and high protein and carb intakes.
Izuku hums happily as she gets to the chocolatey center of her lollipop. Her lips are discolored, a deep cherry red as opposed to the normal pink, and chocolate sticks to her teeth, but she's happy. She has a quirk, a cool quirk, and that means she's even closer to becoming a hero like All Might! And now that Katsuki has his quirk, too, and they're going to be the best heroes ever!
A/N: This is the start of a story, although I feel like it can stand alone. Expect sporadic updates because I have other works that are more of a priority. Please let me know what you think!