Chibimaru-chan, also known as Penelope in English adaptations, is an infant hamster with brown and white fur. Her signature yellow head blanket is ever-present, defining her appearance. As the youngest Ham-Ham, her extreme naivety and innocence often lead her into trouble. She communicates solely through the phrases "Ukyu" (Japanese) or "Ookwee"/"Ookyoo" (English). Pashmina acts as her essential interpreter and emotional anchor, forming a sisterly bond. Chibimaru-chan clings to Pashmina, experiencing jealousy or distress during separation and relying on her during nightmares or when feeling ignored.

Rare instances reveal her appearance without the blanket, such as brief frames in the anime opening, a scene falling from Laura's bag in "Jingle, the Wanderer," merchandise like the Hamtaro Origami book, and manga art. Without the blanket, her visual design shares similarities with Marron in fur color and small size. Their friendship, formed in "Marron’s Autumn Festival," sees Chibimaru-chan assisting Marron in a crisis and boosting her confidence, revealing her supportive nature.

Her development includes forming attachments beyond her species, like Herbert the pig, Ethan (Kylie’s cousin), and a rabbit at Ethan's school, demonstrating her childlike curiosity. In the 1999 OVA "Tottoko Hamutaro: Anime Dechu!", she was initially conceived as male ("Chibimaru-kun") with a shorter blanket. This gender was revised for the main series, where she is consistently female with the longer, familiar blanket.

Episodes highlight her incremental resilience. In "Penelope Helps," she attempts an independent delivery of sweets but gets lost and distracted, requiring covert help from Hamtaro and others to succeed. "Knitting Barrette" explores her separation anxiety when Pashmina is busy. "Penelope’s Dream" depicts her misunderstanding Pashmina’s intentions, causing emotional conflict, reinforcing her dependence while showing growth.