Kemumaki, the primary antagonist, is an 11-year-old ninja from the rival Koga clan opposing Hattori Kanzo's Iga. He operates covertly in a modern city, living alone in an apartment with his ninja cat companion, Kagechiyo. While his civilian identity remains hidden from the public, key figures like Kenichi Mitsuba and the Hattori brothers know his secret.

He appears short with a medium build, sporting tufted black hair and freckled cheeks. His traditional attire includes green robes, a white belt, and a green loincloth beneath his trousers; casually, he dons a blue jacket with gold buttons and lighter blue trousers.

Academically and athletically, Kemumaki outshines Kenichi, yet his ninja skills lag behind Hattori's. He makes up for this with cleverness, frequently inventing and deploying unconventional gadgets in fights. Skilled in dueling, disguise, and various ninja techniques, he uses these abilities to scheme against Hattori and Kenichi, often impersonating them or Yumeko Kawai to cause trouble.

His rivalry with Kenichi is fueled partly by their shared romantic interest in Yumeko. Though primarily disruptive, he occasionally cooperates—helping rescue Kagechiyo or teaching Shinzo water-walking. He shares mutual respect with Kagechiyo, though the cat sometimes chafes at his strictness. Kemumaki staunchly follows Koga principles, once excommunicating Kagechiyo for using ninjutsu to steal ice cream, believing ninja skills should never aid crime.

He drives recurrent conflicts rooted in clan rivalry and personal grudges, yet intermittent cooperation and hints of loneliness add depth. His signature confrontation line is "Barkin' dogs don't bite! I'll get you!", with regional dubs like Hindi adding exclamations such as "Yaai-yaai-yaai!".