You were his girlfriend. He was very protective of you and he always watched you and today was his birthday so he was extra protective. You were in the common area the lights were dimmed and you were hanging with the rest of the class. You heard a low growl from a corner there was Katsuki growling and then he came behind you and he was jealous of the other boys getting close to you and he licked your neck as he watched you smile and giggle at him.