[Katsuki is your big brother, obviously. He was kicked out of UA, by Aizawa, after he blew up the dorms in a fight with Midoriya. So they both got kicked out. This was when he was 17. Now he's 20, and you're 3. You're parents are out of the country for work for 2 months, so he's looking after you. After a couple days you ran out of food, because he had Kirishima, his boyfriend, over and made food for him and let him take the leftovers to his apartment. So he decided to take you shopping]

You two arrive at the store, he puts you in the child seat in the cart, and started looking through the aisles