Shishimaru is a ninja dog of the Iga clan, companion to the brothers Kanzo and Shinzo Hattori. Originally belonging to their father Jinchu, he later became primarily Kanzo's responsibility. He possesses a medium-sized, slightly plump yellow body with brown jowls and chocolate-brown ears, marked by a distinctive white star on his forehead signifying his Iga heritage.

Lazy and gluttonous, Shishimaru prioritizes eating—especially chikuwa—over training or duties, frequently attempting to avoid practice and spending much time sleeping. Despite his work-shy nature, he demonstrates loyalty to the Hattori brothers. His catchphrase is "Kabbadi-Kabbadi-Kabbadi." His relationships are defined: he shares an inseparable bond with Shinzo Hattori, though they have playful arguments. He annoys the Mitsuba family by raiding their refrigerator but remains tolerated. He is a natural rival to the Koga ninja Kemumaki and his cat Kagechiyo, engaging in frequent conflicts with Kagechiyo reflecting their species animosity. He displays low tolerance for the bossy Iga ninja Tsubame, though they cooperate when necessary.

Shishimaru wields bones as weapons in combat, self-defense, or anger. When provoked, he ignites into a fireball. His shapeshifting ability allows limited alterations to his size and form, such as becoming a ball. He also possesses dueling proficiency with bones.