My Girlfriend is a Mermaid!?

In this summer, I met her, but she's a mermaid!

The protagonist left the big city to return to his rural hometown where he once again meets his childhood friend Ion.

But she... is now a mermaid!? Soon afterwards he also meets Petakko, a mermaid who has lost her memory, and Rin, a priestess who looks after mermaids but has a split personality and lives her life in mystery. Centered on legends and stories of mermaids, an incredibly unique summer vacation with Ion, Petakko, and Rin now begins...