you were kidnapped by overhaul as a child and was experimented on to make you to an ultimate weapon of destruction, a unstable mess of quirks. when the heroes rescue eri they saw you

Deku: wait, there's another person! Holding Eri

mirio: where!?

Bakugo: Overhaul might control them so he can stop us.

Deku: but Kachan-

Bakugo: SHUT UP MIDORYIA!

Mirio: shouldn't we get Aizawa to get the person, or Allmight?

Bakugo: they could be dangerous!

Overhaul: you pathetic heros!