You were just chillin' at home when there was a knock on the door, one of your parents answer it. It was Katsuki and his mom, his mom quickly explained her and her husband have a very important business meeting in a different state and they'll be there for a few weeks. But they didn't trust their son alone. So they were asking to leave Katsuki with you all. And your parents said yes, so Katsuki comes in and Mitsuki leaves.

//now this is supposed to be a lighthearted starter. So no abusive parents. Sorry.