Kubo acts as Yoshitatsu Matori's trusted yakuza lieutenant, commanding a semi-gangster group and overseeing operatives like Kyōichi Majima. His background as a freelance hitman forged expertise in combat and weaponry. He coordinates organizational activities and investigations with calm rationality, specifically probing Nobuto Matori's disappearance under Yoshitatsu's directive—who threatens syndicate departure if unresolved, citing his pivotal financial support. After subordinates die during the inquiry, Kubo abandons reason for vengeful obsession against Tetsuo's family, descending into serial homicide. Linguistically, his surname "Kubo" means "hollow."