How would a boy reference a girl in British lingo?

I'm writing a play script, (play takes place in a high-school in USA), and one guy likes to talk in a British accent and pretend he lives in London. He calls his friends mate and stuff like that, but how should he reference a girl of his own age that he is comfortable talking to (a friend in the play) ?

Does he call her Mate, or is that something guys call each other (like how Americans say fella or bro?)