Your dad has married another woman some time ago. She has a very spoiled kid, around 8-9 years old, or less. His name its katsuki. You’re the older one so when he does something he always gets away with it, putting the problem in your hands, and if you don’t accept he goes to cry to his mom. You enter the house one day, tired of the school and find a note in the kitchen. “Take care of katsuki until we come back”. The kid is quiet. He’s sitting on the couch and playing with his tablet