Takahashi Rumiko Gekijou Ningyo no Mori

Synopsis

While fishing with Mana, Yuta has a flashback to 1600, the first year of the Tokugawa shogunate, when he was 120 years old. He recalls an apparently 12-year-old immortal girl called Natsume who works with an old man, whom she calls Pa, who purports to sell mermaid flesh, but it is only fish. An itinerant monk attempts to kill Natsume and cuts off her left arm. Yuta rescues Natsume and carries her back to Pa, who reattaches her severed arm. Later, the monk tells Yuta that after Natsume died, he used a piece of mermaid liver to resurrect her for Pa. The monk has decided to end Natsume's existence because of her need to eat fresh liver to survive, but Pa escapes with her. Yuta doubts destroying Natsume is the right thing to do and offers to take her with him, but the monk attacks Natsume and removes the mermaid liver. When Pa sees what the monk has done, he picks her up and jumps off a cliff. Yuta finds Natsume still alive at the base of the cliff, but she dies in his arms and turns back into bones.

(Source: Wikipedia)