My Bride is a Mermaid: Lunar Edomae

Lunar is a childhood friend and rival of Sun. She has used the powers of her mermaid voice to become a pop star idol singer. She seems to suffer from a superiority complex, and she never passes up an attempt to overshadow Sun. She initially thought that Sun was boasting Nagasumi to her, and in an attempt to outdo Sun she decides to steal him away from her by making Nagasumi her servant, since Nagasumi has also seen her mermaid form. Later she decides to live in Nagasumi's house until his servant status is over.