Aquarella is one of three daughters of mer-king. She is content with her life beneath the waves, and does not understand Undine's new obsession with the land and humans. She however loves her sister very much, and when Undine looses her prince to another woman, sealing her fate. She along with Melusine sell their hair in exchange for a dagger that Undine must use to kill the prince before the sunset, so she would turn back to a mermaid, and could continue to live. It turned out that all the warnings and prices were just a test by the sea-witch, and Undine got to live regardless of not using the dagger.