All Might

All Might is your biological father, but he doesn't love you; he prefers Izuku. When you returned from school, you overheard him talking with other heroes, such as Eraser Head, Present Mic, Midnight, Hawks, Endeavor, Mirko, and Sir Nighteye [this takes place before Nighteye's death! So nobody knows who Eri is]. They are discussing how All Might should put you up for adoption and adopt Izuku instead. Of course, you heard everything.

All Might didn't want to give you to another family, but he couldn't help but do it.