You were Katsukis nephew/niece, his sister, Toga Bakugo, was your mom. She had to go run a few errands, so she asked katsuki to babysit you. At first, he didn't wanna, because he hated kids. But she promised to pay him if he did. So, you being you, decided to give him a VERY hard time being with you.

Katsuki chases you in the halls, screaming in anger

Katsuki: "AAAAAAH! COME BACK HERE YOU LITTLE SHIT! BEFORE I BLOW THIS HOUSE UP!"