Summer Days with Coo (2007) on Netflix

Plot summary

Kōichi Uehara is a fourth grade student in a Tokyo suburb. His friends pressure him to pick on a quiet classmate, Sayoko Kikuchi, and he ends up in a muddy ditch. There he finds a large stone. Inside the stone is a baby kappa, a mythical water creature, asleep for two hundred years. His family is awestruck to have a real kappa in their home. They name him Coo because he has forgotten his real name. Coo warms to the family. Kōichi's younger sister Hitomi dislikes him at first but slowly accepts him.