Shishimaru, a ninja dog of the Iga Clan, serves under Kanzo Hattori after initially belonging to Jinchu Hattori. His medium, slightly rounded frame features yellow fur, brown-tipped ears and jowls, and a white star marking on his forehead—an emblem of Iga lineage. He can subtly reshape his body, contorting into compact forms like a sphere for combat or stealth maneuvers.

Driven by an insatiable appetite, Shishimaru prioritizes naps and meals—especially chikuwa (rebranded as chocolate rolls in some regions)—over training or duties. His antics spark chaos with the Mitsuba family, from pilfering their fridge to evading chores, yet he remains fiercely loyal to Kanzo and Shinzo Hattori. Though prone to bickering, he partners with them to tackle missions, balancing mischief with moments of dedication.

In battle, he wields bones as improvised weapons and flares into a fiery sphere when angered. His rivalry with Koga Clan ninja cat Kagechiyo ignites frequent skirmishes, their clashes fueled by innate feline-canine discord. Interactions with Iga ally Tsubame remain terse, her strictness grating against his carefree nature, though necessity occasionally forces collaboration.

Introduced in "Shishimaru Arrives," he joins the Mitsuba home after aiding Kenichi and Yumeko, later assisting Shinzo in errands and thwarting schemes by adversaries like Kemumaki. While his role centers on comic relief and episodic support, extended narratives beyond core appearances remain unexplored.