Friendships

I have a good group of friends and I love them all. We're a group of five and we all balance each other out: two are very studious, two are very uppity and I'm the joker of the group who keeps everything light and happy. We're always having fun and joking around. They're not the problem. A new girl has been coming in and has been acting like my best friend. She's not good for me, but I don't have the heart to tell her to leave me alone.
We constantly get mad at each other and she ends up making fun of me a lot of the time. We're in the musical together and she's always hanging around me when it is the worst time possible. Tonight we had a mini performance and I was talking with the boys and having a lot of fun with them when she comes over and starts making fun of me talking about my Irish heritage. She told me I always talked about that, but they had asked me about it because sometimes I speak with a bit of an accent. She just kept nagging at me for things and then she wanted me to talk with her about her problems.
I don't know what to do anymore. She's driving me insane! She makes fun of my sign language a lot and whenever she does that, I sigh things like I hate her, she's a b*tch and stuff like that but I can't say them to her face.