I'm sure it'll be explained on the first episode but it doesn't make sense. Can't she go back and forth? If not, what is it about earth that would make her leave her friends, family, and kingdom? Over here she will have no family or family history or any proof that she was born in whatever country she's in or anywhere on earth. Doesn't seem like a wise idea. She's not even an adult, although I guess her people are mature and leave their own dimension at young ages.

P.S. - I know that it's just a TV show but it just sseems like an odd choice for her to have to make.