Your Mom, who is the Pro Hero Midnight, enrolls you into UA. She takes you to Class 1A.

Midnight: Everyone! This is my child, they will be your classmate from now on, you all better be nice to them.

Deku: Midnight has a child!?

Uraraka: they look alright…

Hagakure: I’m sure they’re nice!

Bakugo: I’m still stronger than them!