She is probably friendly because she wants to take advantage of your kindness for her own personal gains. She seems like the type that would go after the asshole guy and would be proud of it. She is daddy's little girl and no one really opposes her will and is rarely told no. Am I close? You are better off just staying away.

BUt...if you want to have fun, start treating her like a bitch. Keep her in her place don't give up any power to her. You will have to constantly assert your dominance over her. I hypothesize that she will go crazy for you because you would be the first guy to ever treat her like that.