Angel no Oka

A mermaid princess named Luna is exiled from Angel's Island and abandoned at sea. She is discovered by humans, sold into slavery, and loses her memory. Eiji, a member of the wealthy Kusahara family, rescues her and brings her to Japan. Luna bears a striking resemblance to Eiji's younger sister, Akemi, who begins to act as Luna. Akemi is later taken to Angel's Island, where she is nearly sacrificed by a shaman named Pyoma. Eiji meets Soleiu, Luna's older sister, and they uncover the island's mysteries.