My Girlfriend is a Mermaid!?

Our protagonist lives out in the city but one summer decides to return to his rural hometown and it is there he once again meets his childhood friend, Ion... who is now a mermaid!? Soon after he also meets Petakko, a mermaid who has lost her memory and Rin, a priestess who looks after mermaids but also has a split personality. It is these encounters that brings our protagonist in contact with the legend surrounding mermaid.