Hoshi-kun, a celestial being from the sky, manifests as a small star that descends to Earth and befriends the protagonist Cat. After landing, he takes shelter inside Cat's guitar, forming a close bond that initiates their friendship and anchors the story "Cat and Star."

He engages with fellow characters—Rabbit, Bandit Cat, Bird, Squirrel, and Mr. Moon—participating in group activities like singing to Mr. Moon. Hoshi-kun joins shared adventures such as ice skating, flying on a blanket, and riding a whale, often alongside Cat and the ensemble.

The character appears across multiple series entries, including "Singing to Mr. Moon" and "A Snow Spirit on a Snowy Day," where he partakes in seasonal or whimsical group pursuits. These stories consistently position him within the central cast, contributing to episodic, gentle narratives.

No evolution or expanded backstory beyond his falling-star origin occurs in any documented media—OVAs, theatrical releases, or adaptations like the 1993–1996 video games for Super CD-ROM², 3DO, Pippin Atmark, Macintosh, and Windows. Hoshi-kun remains a constant presence in the ensemble throughout all official appearances.