During her summer holiday at her grandparents house Tamai Shiina, a young and cheerful schoolgirl, meets a strange looking creature. They befriend each other and Shiina names it "Hoshimaru: The Round Star." When Shiina returns home after the summer to go back to school, she starts meeting other kids that also have befriended a strange creature like Hoshimaru. But she soon finds out that not all these creatures and their masters are as friendly as Hoshimaru.
Summary taken from: MyAnimeList