The Twelve Kingdoms, Volume 1: Sea of Shadow

Yoko Nakajima, a typical, obedient Japanese high school student, has a fairly ordinary life--that is until Keiki, a unicorn in the guise of a young blond-haired boy, tells her that she is his master and must return to their kingdom, but when the boy mysteriously vanishes, Yoko is left alone on a quest for survival and self-discovery.