my current thought is that a group of her people came to earth in ancient times either because their planet was dying or they were exiled or they werre studying humans (probably the last one). a small group realized that with their powers would be worshipped as deities and took those roles. eventually... the population dwindled either through exodus, illness, uprising or returning to their homeworld, she was left behind. she has moved through the centuries, others like her have had small cults around them.
powerwise, i am thinking immortality, and either reality warping or some variety of manipulation (an element, weather, healing, pheromones, etc) just not sure which one. i am playing it loose at the moment.