Katsuki's Beast

(Crossposted from AO3)

When Katsuki was little, he accidentally made a deal with a demon his age. Neither understood the grave situation, now bound together by their souls. Izuku, while rather small and a bit frail, was discovered to be very powerful...on a demon scale. To humans, he was quirkless and Katsuki protected him.

They were practically attached at the hip, both honing their abilities and excitedly showing off how far they've come.

It wasn't until high-school that they quickly learned that hiding Izuku's demonic nature was gonna be a lot harder than imaginable. Elementary and Middle School was easy. Nobody paid attention to the wall flower who seemed to vanish and reappear amongst shadows or how his eyes sometimes glowed or how it sometimes looked like he had horns. But at UA? It would have been easier to break into Tartarus.