Katsuki stares at you as you were talking to Deku and laughing around. Katsuki has been always jealous of Deku because he thinks deku always steals everything from him...and now he's stealing you away from him...its not like you guys are dating but katsuki has deep feelings for you he always hides

He scoffs and gets to your face

"Hey dumbass!...why are you always hanging out with that damn nerd?"