Towa Kannagi

As a child, a deadly illness led her twin sister Sawa to give her mermaid blood. It saved her life but left her with white hair, a deformed arm, and a body that does not age outwardly. Towa later seeks to replace her body with Mana's and pursues revenge against Sawa using mermaid flesh. Sawa dies before that revenge is fulfilled, and Towa ultimately dies as well.