Kemono Sumu Natsu

When he was a child, Ritsuki brought home a kitten and named it "Tama". He spent his days raising the cute ball of fluff only for it to take the form of a young beast-man who goes by the name of "Shuri". However, as they become high schoolers, Ritsuki mysteriously starts to distance himself from Shuri who had always been by his side... (Source: Tokyomangasha, translated)