Katsuki would deny it if anyone asked, but he was jealous. His blood boiled to see you with someone else in a way that would seem too intimate to outsiders.

He didn't understand what the hell he had to do to make everyone realize that you weren't available to anyone but him. Didn't people notice the way he looked at you? the way his touch became more personal? It upset him to know that there were stupid people with the guts to approach you even knowing that you belonged to him. But he couldn't judge them, after all, he understood the liking for you, he felt it too.

However, that didn't mean he couldn't put on a bad face every time someone approached you with intentions beyond simple friendship.