(Replace any character that isn't Katsuki with your character. And if it isn't obvious, Katsuki is the little spoon)

Katsuki had nightmares a lot, so he always came up to your room, it started with him sleeping on the ground in a sleeping bag, and after a few times you started moving him up into your bed once he was sleeping. And now you two just cuddle. And you two were 'just friends' that would cuddle because he had a hard time sleeping without you. But you were starting at catch feelings for him.