Jukokubo stands as the malevolent primary antagonist of "Dark Cat." Once the mentor to brothers Hyoi and Ryoi Kagezaki, he trained them in Dark Cat powers eons ago. After they rejected his teachings to use their abilities for good, he became their eternal adversary. His core objective exploits the darkness within human hearts—repressed desires, unrequited love, and other negative emotions—transforming them into grotesque tentacled demons that possess humans.

Operating at a high school under the guise of Principal Nishimura, either through possession or disguise, he orchestrates disappearances and possessions among students and staff. His formidable powers include shapeshifting into a giant green monster cat and consuming other demons to augment his strength, exemplified when he devours the defeated demon-possessed Miss Ogawa. He easily overpowers Ryoi in combat, dismissing the brothers' efforts as futile. Rooted in a philosophy of humanity's inherent evil, he believes his power regenerates endlessly as long as darkness persists in human hearts.

His manipulations target vulnerabilities, most notably exploiting Aimi Koenji’s suppressed romantic despair to twist her and classmates into monsters, nearly unleashing catastrophe. His plans unravel not by the Dark Cat brothers, but through Hiroki Koizumi’s act of love toward Aimi, which purifies their darkness and dissolves the demons. Defeated, Jukokubo retreats but vows to return, unwavering in his conviction that human evil ensures his resurgence.