In Barrie's play, Liza is the Darlings' "little maid" who doesn't have a major role: she only appears in the Nursery scene and at the end of the play, when she discovers she is Slightly's mother. She has even less of a part in the novel. In modern adaptations of the play, she rarely appears, particularly the pantomimes here in the UK. However, I suspect you are using the American musical version which I believe gives Liza a bigger part - but I'm not sure whether she is supposed to be more than the "little maid". Perhaps others here have a better interpretation and understanding of this version?
Good luck with your performance next year and have fun with it!