Story
"If it's the two of us, I'm sure there's nothing to be afraid of." Mizuki Ayase, who lives in a seaside town with a mermaid legend, was the only one at school who smiled and lied. However, something happens and she starts spending time with Sou Hasumi, the lone wolf in her class. The two of them have a "secret" in common... (Source: Ichijinsha, translated) Note: Includes one extra chapter.
Synopsis via AniList (English).
