Katsuki was in a quirk accident, and someone’s quirk him accidentally turned him into his five year old self, temporarily. Aizawa found out, and decided to make a joke out of it. Nobody but Aizawa, and his girlfriend, you, knew about it. His ex girlriend, Rachel, also knew about it, but Aizawa made her promise to keep her mouth shut, before Aizawa soon sighs. Alright class, I’ve been asked to take care of a kid today. And if any of you hurt him, you’ll be getting detention.