Kemuzō Kemumaki is a ninja from the Koga clan and serves as the main rival to Kanzo Hattori, the protagonist from the Iga clan. As a student at the same school as Kenichi Mitsuba and Yumeko Kawai, Kemumaki is depicted as being short and stocky with spiked black hair, freckles, and a signature green ninja outfit, though he also wears casual clothes like a blue jacket and trousers. His personality is a mix of brash ambition and hesitant insecurity, often becoming overconfident in his schemes only to have his plans backfire due to miscalculations or when he is outmatched by Hattori's superior skills.

Kemumaki is primarily motivated by two intertwined desires: to prove the superiority of his Koga ninja skills over his Iga rival Hattori and to win the affection of his classmate Yumeko, which puts him in direct competition with Kenichi. This rivalry is fueled by a historic feud between the Iga and Koga clans. He often devises intricate schemes involving elaborate disguises, deceptive ploys, and advanced gadgets to outmaneuver both Kenichi and Hattori. For example, in his first appearance, he pretends to be a polite and kind new student to impress Yumeko and deceive Kenichi's mother, only to reveal his true antagonistic nature later. In his role as the primary antagonist, he is responsible for most of the bad events that occur in the series, constantly trying to cause trouble for Kenichi and his friends.

Despite his often self-serving and antagonistic nature, Kemumaki sporadically reveals a more vulnerable and even cooperative side. He has been known to collaborate with Hattori to save his beloved cat, Kagechiyo, and has mentored Hattori's younger brother, Shinzo, in techniques like water-walking. On one occasion, after being trapped in a dog disguise intended to trick Yumeko, he briefly embraces the ruse and its comfortable perks until he is rescued. His key relationships include his sharp-tongued but loyal feline partner, Kagechiyo, who is not afraid to challenge his master's harsher impulses. While he is a nuisance to Kenichi, he is perceived as polite by Kenichi's parents, Mr. and Mrs. Mitsuba, who are unaware of his true, troublesome nature. He lives alone in an apartment with his cat, and no family members are known.

As a ninja, Kemumaki is adept in ninjutsu, combat, and mimicry, frequently using disguises to impersonate Hattori, Kenichi, or even Yumeko to sow discord. His overconfidence, however, often leads to his own downfall, and he is known to be easily intimidated by some of Hattori's more powerful techniques. While he constantly challenges Hattori to duels, the Iga ninja almost always emerges victorious.