Lu Over the Wall(2017)

Kai Ashimoto, a gloomy young middle schooler, lives in Hinashi Town, a lonely fishing village, with his father and his grandfather, a parasol maker. He used to live in Tokyo, but after his parents divorced he moved back to his father's home town. Kai has trouble telling his parents the complicated feelings he has for them, and he's lonely and pessimistic about his school life. One of his joys is uploading songs he writes to the internet.

One day, his classmates Kunio and Yuuho invite him to join their band, "SEIREN". As he reluctantly follows them to Merfolk Island, their practice spot, they meet Lu, the mermaid. Lu sings merrily and dances innocently. As Kai begins to spend time with her, he starts to be able to say what it is that he's really thinking.

(Source: Wikipedia)