Chibimaru-chan is a baby hamster with white and brown fur, almost always wearing a yellow blanket over her head. Owned by the human child Kylie, she is the youngest member of the Ham-Ham group. Her name translates to "little round one," reflecting her small size and youth. Communication primarily consists of vocalizations like "Ukyu" in Japanese, adapted to "Ookwee" or "Ookyoo" in English dubs, mirroring her immature development.

In the OVA "Tottoko Hamutaro Anime Dechu!", she is called "Chibimaru-kun," with a shorter blanket design compared to the main anime series. Her yellow blanket remains a key visual element, though she appears briefly without it in the English dub's opening sequence, the episode "Jingle, the Wanderer," and the game "Hamtaro: Ham-Hams Unite!".

Her personality centers on childlike innocence and naiveté, often leading to trouble. She forms a deeply attached, sisterly bond with Pashmina, exhibiting jealousy when Pashmina is occupied, as seen in episodes like "Watching Over Cute Penelope" and "Knitting, Knitting Barrette!". Later narratives show her affectionate tendencies toward non-hamster characters like Herbert the Pig, Kylie's cousin Ethan, and a rabbit at Ethan's school. In spin-offs like the "Tottoko Hamutaro Hai!" series, she engages in age-appropriate activities, such as assisting Pashmina with lunch delivery in episode 31.

Beyond Pashmina, she develops a notable friendship with Marron during "Marron's Autumn Festival," where they collaborate to prevent river flooding. Their connection is reinforced by similar height and fur color, with Marron resembling Chibimaru-chan without her blanket. Background details vary: early manga-based material presents her as Bijou's younger sister, while later sources describe her as Pashmina's sister. Her communication evolved subtly, with the English dub shifting from "Ookyoo" to "Ookwee" to avoid unintended phonetic similarities.