A Magical Girl trope. Usually more heavily linked to the Cute Witch, especially travellers from other realms or planets, than other types of magical girls. Basically the protagonist is a magic user who is a candidate to become Queen back in her home realm/planet/whatever. She has come to Earth as part of a test of character to prove she is queen material, either by making the world a better place with her magic, or by directly competing with a (usually friendly) rival (who often is at least somewhat of a Dark Magical Girl) to accrue some sort of Plot Coupon to prove her worthiness. That or it is just tradition for magical princesses to spend some time on Earth before they can become queen. While doing so she usually lives with some Muggle Foster Parents who may or may not know she is magic