Katsuki gets her quirk when Izumi is three and she herself has just turned four. It comes with the smell of burnt sugar, in a wave of popping sparks that light up her friend's face as she studies her palms in wide-eyed awe. It's a look that strikes at something within Katsuki, and she realizes that she never wants people to look at her any other way.
Then she looks up, and mixed in between the looks of admiration and awe from her classmates are the tight-lipped smiles from her teachers. One of them looks a little hopeful, but the other two have suddenly closed off.
It takes Katsuki aback. She's always been the strongest, the fastest, the smartest, the best in her class. Her teachers have never been anything but praising towards her, and her classmates have always known that she belongs at the top of their beginning pecking order. This is the first time any of them have ever looked at her with something bordering disapproval.
"Kicchan!" Izumi burst out, unable to handle silence any longer. "You're amazing! Your Quirk is so cool! It's perfect for hero work!"
Katsuki turns her gaze from her teacher's disappointment to Izumi's excitement, and something else clicks. She sets off another round of mini-explosions in her palms and stares at them thoughtfully. After a moment, she looks up at Izumi with a proud smirk.
"Of course it is," she sniffs with all the pride a four-year-old can muster, before looking her teachers right in the eye. "I'm the best."
Izumi doesn't come to school for four days in a row, and Katsuki is sick of it. She stomps home after school with a glower on her face and stalks up to the Midoriya household. She pounds on the door, and when Auntie Inko answers it with a perplexed look, slips past her with a scowl.
"Izu!" she yells, followed closely by Auntie Inko.
"Katsuki, honey, I don't think -" she starts, hesitantly.
"Izumi! You can't hide from me!" Katsuki shouts, peering into each of the rooms while ignoring Inko's words.
"Katsuki, you really -"
She stomps up to Izumi's room and slams the door open. There's a lump on the bed, tucked up under a pile of blankets, unresponsive to Katsuki's presence. Katsuki studies it for a moment before bounding up onto the bed and kneeling beside the lump. She mercilessly strips the blankets away,
Beneath them, Izumi is curled into a ball, eyes dull, ringed in red, and lifeless. Katsuki blinks at the sight of her, momentarily startled out of her irritation. In the ensuing silence, Izumi speaks, her voice a hoarse croak and as dull as her expression.
"I'm Quirkless," she says softly, and it feels like a punch to the gut. "Mom took me to the doctor. I'll never have a Quirk. I'll never be a hero."
Katsuki doesn't have an immediate response to that. She hadn't expected this when she first showed up - expecting some stupid reason to be the cause of Izumi's absence. Nothing like this. Nothing that would throw such a big wrench into their plan to be the Wonder Heroes together. She considers her friend for a moment.
"So, what? That's it?" she scoffs. Izumi blinks slowly then tips her head to look at Katsuki. "That's all? You got some bad news, so you're just going to roll over and give up? Just lay down and stop trying, because what's the point?"
"I don't…" Izumi says, a look of growing confusion on her face.
"You don't have a Quirk," Katsuki says harshly. "According to the doctor, you never will. Are you going to let that define the rest of your life, or are you going to get up and do something about it? Are you that weak that you'll give up your dreams at the first - the first villain you fight?"
Izumi blinks at her some more, but underneath the confusion there is growing life, and Katsuki knows her attempt was successful.
"But how can I do anything if I… I don't have one?" she asks, uncertain.
"How am I supposed to know?" Katsuki retorts, sitting back on her heels. "You're the hero nerd. Weren't you going on the other day that what's-his-face, y'know, Man Bun, was fighting basically Quirkless since his Quirk isn't good in combat?"
"You mean Cryptogram," Izumi laughed weakly. "His Quirk is language translation and is named Rosetta after the Rosetta Stone. It doesn't do much in a villain fight but is useful in other ways and is such a cool Quirk because -"
Katsuki quickly covers Izumi's mouth with her hand. Once the other girl gets started, there's no stopping her. Izumi shakes her hand off before sending Katsuki a sheepish grin. Katsuki just rolls her eyes in response.
"Sorry," she says, not sounding very sorry at all. "I think I get it. I just need to do what Cryptogram and the others have done, and I can still be a hero. That's what you mean, right?"
"Right," Katsuki nods decisively, ignoring the fact that neither of them knows what exactly it is that heroes do to actually become heroes. "Now come on. It's time for dinner, and I know Auntie's been worried enough about you that I think I can convince her to make me her Lava Tantanmen as thanks for dragging you out of your misery."
"Kicchan, you're the only one who can eat that! It's too spicy!" Izumi's complaints fall on deaf ears, but it's okay because she's smiling and the complaining is more habitual than genuine anyway, so Katsuki ignores her.
When they reach the bottom of the stairs, Auntie Inko looks at them in surprise. She gazes at Izumi and her smile for a long moment, before tears fill her eyes and she turns aside to blink them away. As Izumi relays their request for dinner (including the Lava Tantanmen, Katsuki notes smugly), Inko turns to Katsuki and gives her a grateful look.
All throughout dinner, Katsuki squirms uncomfortably as Inko ruffles her hair, gives her an extra-large helping of Lava Tantanmen, and looks at her gratefully. Katsuki didn't do anything that special, and even if she did, she didn't do it for Auntie Inko. Katsuki's not even sure that she would have done it for Izumi. So she really shouldn't be getting thanked for something she did for herself.
Still, the next day when Katsuki leaves her house to go to school, Izumi is waiting at the corner as usual, and they head off to school together.
A week later one of the boys at school somehow found out about Izumi's Quirklessness and spread it around the school. A few of the meaner kids get wind of it and decide to corner Izumi after school.
When Katsuki goes looking for her, she arrives just in time to see one of them push Izumi to the ground, spewing off some crap about heroes, girls, and Quirks. Katsuki doesn't really remember because her vision goes red and when it finally clears, she's thrown her first punch, been in her first fight (that she obviously won), and is waiting for her parents to arrive with the wannabe-bullies and Izumi in the principal's office for the first time ever.
Izumi's hands are scraped from her fall, and she looks like she's about to cry, but the relief Katsuki had seen in her expression when she'd arrived keeps her head high as she stares down the crowd of adults that slowly trickle in.
The next hour is filled with glaring, yelling, and children's tears. Katsuki remains unrepentant through it all, and once Auntie Inko and her old hag realize what the fight was about, they round on the school officials and other parents with a vengeance that would be alarming in any other circumstance.
The situation repeats itself a few more times over the next several weeks, but by the end of the month the rest of the school knows two things. The first is that Bakugou Katsuki is top dog at the school, and anyone who challenges her will end up in the dirt. The second is that Midoriya Izumi is off-limits.