Angel no Oka

"Angel's Hill" is a fantasy story depicting the stormy life of a mermaid princess, Luna.

Luna is put into a seashell and floated out to the sea because she breaks the rules of the Angel's Island. She is then rescued by humans aboard a ship but sold as a slave. She loses her memory and is treated very badly. Then comes Eiji, the son of the rich Kusahara family. He saves Luna and takes her to Japan. Surprisingly, Luna and Eiji's younger sister, Akemi, look as alike as if they were twins. Akemi, who has a perverse nature, turns into Luna on a whim. Akemi is kidnapped and taken to Angel's Island with Eiji. There, Akemi is almost offered as a sacrificial offering by a female shaman, Pyoma. Meanwhile, Eiji meets Soleiu, Luna's older sister, and together they get involved in the secrets of the island.

(Souce: tezukaosamu.net)