Overview
CV: Satsuki Yukino
Hoshimaru (ホシ丸) is one of the dragon children, whom Shiina Tamai met. Hoshimaru is shaped like a star. Its name comes from a comment made by Shiina that "stars are round" (hoshimaru means star circle in Japanese).
It can change any part of its body except its face at will, and it transformed into a bag to accompany Shiina to school and other daily activities.
Compare to other Mohiro Kitoh's creature designs, Hoshimaru has a much more moe look, so there is a few fan art of them on pixiv.
Dragon Bearer
The person linked to Hoshimaru is not Shiina, but actually Takeo Tsurumaru. The anime version of the story ends before this is revealed.
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Narutaru Shiina Tamai Takeo Tsurumaru Dragon Child Star