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It had always been fire. When he was smaller than he had ever been before, his father breathed fire and the world turned red. Fire was shouting and threats and that awful smell, and fire was panic and hiding and trying to cry quietly because his father hated hearing him cry. Even when he left, even when three plates at dinner became two, he hoped desperately he got his mother's quirk just as he had her hair and her eyes. He didn't want to think about having to relearn the heat and the red light. He didn't want to force his mother to carry the weight of his father for the rest of his life.
When he was just a fraction larger, his best friend grew embers in his small hands. Tiny bombs- it was amazing, the blasts of heat and light, until suddenly he stopped being Izuku and started being Deku and he could never be a hero and his best friend buried his embers into the flesh of his shoulders and Izuku-Deku watched as his entire world burned to ash before his eyes. He sobbed as the flames licked his arms and left bruises the size of a child's fists.
There is a fire on the inside, too. His core burns bright as a star- he will be a hero. It's as much a part of him as his lungs, his heart, his eyes. Sometimes he wonders- if he didn't burn so brightly, if he didn't fight so hard to stand on the same ground as his peers, would he still get burned? He is young, and he is trying so very hard to understand what he did wrong, so maybe he could fix it and Kacchan would be his friend again. He just wants his world back. He is young, and his lungs are filled with ash ash ash, sometimes so thick it clogs his throat, and his mother teaches them both to speak with their hands so he has a refuge on the days the air is too thick with smoke to breathe.
He gets larger yet. He is stick-thin limbs but a soft face when Kacchan breathes fire and his words strike like matches. Sparks burrow in freckled skin and ricochet between notched bones- a leap of faith from the roof. Deku-Izuku shakes and wishes there was enough of him left to breathe fire back, but his core of sun-stuff is buried in layers of hide and bow and bend and flee, because those are the only ways he knows to dull the flames around him. He has been afraid of the flames all his life.
He is not any larger at all when he meets a fire he's never seen before- the type that warms and casts light like a spotlight, but somehow does not burn. All Might's fire scorches the villain out of Izuku's lungs, and he can't help but gape at a power that licks at his skin and does not leave wounds behind.
All Might makes him larger, stokes the starfire in his core with hands he won't stop holding out to him. Even on the days he just wants to collapse in on himself, those hands stay stretched towards him and it makes Izuku want to cry. Sometimes it does make him cry, and he feels Deku washing up his back, but All Might smiles on and doesn't turn away (not like every adult he's ever been under, who saw quirkless before anything else, who saw useless before anything else) and Izuku settles back into his skin with a happiness that weathers through burning lungs and ribs and screaming muscles.
He is larger when he shows up at UA for the first time. Muscle and promise wraps around bones thicker than they once were, though they still hold tales of old fire. The exam looms over him like a titan, and an utterly alien fire is lacing through his human marrow. Deku comes crashing in with the face of a friend-turned-demon and a girl who smiles like a flashlight. Deku does not get a single villain point, but Izuku cannot watch someone in need and do nothing. It's an all-consuming inferno that devours his limbs and annihilates the zero-point enemy, and a fire burning him alive as he lays with three shattered limbs on the black of the road. Failure burns almost hotter and it's Deku Deku Deku who walks home with a hung head.
The fire eats away at him until All Might himself fills the room with light, that ever-burning presence overwhelming even through the hologram.
His sun core feels brighter after that, less like desperation and more like raw determination.
It's a feeling like embers in his guts when he walks into the room, because he knows that hands that reach for him are filled with fire, and the hurts in his bones were given by kids smaller and weaker than the teenagers strong enough to get into UA. And hands do reach for him- and they spill light, but they do not burn. They aren't the all-encompassing sunlight of All Might, but Deku-Izuku marvels at their painless warmth.
Fire is the core of a hero. The drive to rise, to save, to push forward at every cost, is integral to heroism. But for once in Deku's life, Deku doesn't mean worthless. He is Deku- full of fire. His fire used to burn him, draw hurt like moths, but here, there are no sidelong glances and ugly mutters. No fists strike out to harm (outside of training), and no hands reach out to yank his hair or steal his things or destroy destroy destroy. He sees open palms, and he finds cores that slide like puzzle pieces next to his. The feeling is utterly intoxicating.
The fire is still there, even on the good days. His eyes may have forgotten, but his notched bones have not. He still flinches at outreached hands sometimes. His name called out sometimes draws his heart to fever pitch. But he is peeling away, layer by layer, all the things that stand between his sun core and the world he has pledged to save. He was passed a torch- he is Deku, who burns brightest, and he will bring its light to all the world.