Most people who are born with the X-gene do not know it until they hit puberty. Unfortunately, Marigold's X-gene presented from the moment she was born. All she has known her whole life are the white walls that surround her. No one knows of her existence, until someone makes a call to Professor Xavier and his X-Men team. ( First chapter takes place between episodes 2 and 3 of season 1. So Kitty is already apart of the X-men, but this is before Rouge shows up.)