Dealing with someone who has too much personality...?

I need some advice..my boyfriend and I have a great relationship. There's only one minor problem. His roommate's girlfriend has a HUGE personality and makes sure to turn a conversation to be about her. You see, she flirts with my boyfriend like crazy, however he takes none of it.. she's very promiscuous, lets just say she's a little party crazy and she tries to boss me around because, yes I am a people pleaser but I want it to stop with her. My question lies in how do I deal with a girl who is three years younger than me (very immature) but still keep composed and not so bothered about what she does. I want to be superior by my age level. I Whenever I am around a group of people I don't know, I generally am more reserved however, with this girl she is just always up in your face. I don't want to feel like I need to be more outgoing (or crazy like her) to be noticed. It's really hard not do to anything when she hits on my boyfriend and tries 2 make me feel inferior to her.