This girl has a lot of very complex psychological issues, stemming from god knows where. You are in no position to help her, and in fact, she is likely to drag you down, too. You say her grandma yells at her, but perhaps that is because her grandma recognizes what poor decisions she makes, and is trying to stop her before it is too late to fix her mistakes. If I were you, I would run far, far away, and stay away from this girl. She is heading down a dark path.
And BTW - I hope it was just a poor choice of words on your part when you said she wouldn't mind you.
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?