A backdoor girl. One who can't be trusted. A girl who is all about opportunity and doesn't care who gets hurt. Her feelings are never real and turned on and off based on "what's in it for her". As a friend, you can never confide in them, because if it suits them, they will betray you without a second thought. As a girlfriend, well let's say, you are in for an emotional roller coaster and she at the controls. But it won't be long before she finds what she thinks is a better ride and you will be conveniently shelved, to fuck with later. Dont bring her around any of your successful friends unless you are trying to get rid of her.
"Wow, I just saw your boyfriend with Sarah, I thought she was your best friend? What a trifling bitch!"