Boku wa Ningyo Hime wo Tabetai

A descendant of a mermaid, Amami Riza, has obtained a jewel that holds the power of a mermaid. On the day she transformed into a mermaid for the first time, her classmate Kanda Ruka found her and asked her for mermaid tears, which are said to cure anything. In fact, Ruka was under a curse... (Source: Ciao Plus, translated)