Telling someone outright you dislike them?

I'm wondering what's the worst that could happen if I tell this girl outright that she's annoying and I don't like her. She's kinda my friend, but not really (as in she thinks we're friends). She's not very popular because a lot of people think she's annoying, but no one's really said it to her face. She kinda creepily obsessed with this guy who she says she doesn't like, but she still won't stop talking about him. And she says that she does like kickboxing, etc. and has friends who can help her beat people up (she's never directed that threat at me) but I'm not sure if she's just bluffing or not. It's getting to a point that I really can't stand her. So I was wondering what's the worst that could happen to me, socially and physically. Any thoughts, guesses, speculations?