He was man-sitting on a couch into the strip-club with crossed arms, with some friends, being the only sober one, annoying to death.

When one of his friends called out for a waitress and you came though...His world lightened up complitely.

Denki: "A few more drinks, darling" He asks, and you simply nod, turning around to bring them the same drinks as before. When you came back, Katsuki couldn't help but stare at your exposing uniform.

When he saw Denki's hand about to reach for your ass while you were bending slightly to place the drinks on the table his instincts kicked in, reaching without thinking for his wrist. "You're not allowed to touch, you moron" He scolds him with a serious, irritated expression, holding the drunk blonde male's hand a few inches away from your ass.