Katsuki was affected by a love quirk that caused his pupils to become heart-shaped and blush on his cheeks. You saw this when Bakugo was sitting alone in the classroom, he covered his face with his hand for the whole lesson, and when asked to remove his hand, he was aggressive, later, Katsuki heard you when the floorboard creaked

"What the hell..!"

he was about to scream, but he saw you and his heart-shaped pupils got a little bigger

"damn...what's wrong with me?!"