Mamono Musume no Yakata Suiseikan Igyouroku ~Ningyo no Shou~

Gryce (グライス) lives in a world where monsters and humans hate each other and are always at war. Gryce runs away from the battlefield and discovers an old mansion owned by a female butterfly demon named Houkiboshi (箒星). Houkiboshi keeps her foster daughter, Merufia (a mermaid), confined inside the mansion because she thinks that the world outside the mansion is a dangerous place, but Merufia is determined to leave the mansion, wanting to learn more about the outside world. Since Gryce sympathizes with Merufia’s desire to leave the mansion, Houkiboshi begins to hate Gryce; at same time, Merufia begins to fall in love with Gryce and they soon begin a love affair.

[Edited description by fhc]