Natsu ni Kanaderu Bokura no Uta

When Haruka was small, he was friends with many girls. One day, those girls played a secret game amongst themselves and one of them never appeared after that game. Many years later, while enjoying the sea, he meets a girl who he thought was a mermaid. She was Kanon, the girl who had left them back then. He calls out to her, but she doesn’t acknowledge knowing him.