yotsuba is actually an alien and has superpowers. when she turns 19, those powers manifest themselves and she finds that she can fly and shoot beams out of her palms. she then decides to be a crusader for justice, aiding all who require her.
also, later it's reveiled that koiwai is an alien too, and he was sent to earth to protect yotsuba while her powers developed.
series ends when yotsuba saves the earth from yanda, who was coincidentally her alien tribe's arch nemesis all along.