猗窩座 (Akaza) is the demon Kyojuro Rengoku faces and is killed by at the end of the Mugen Train arc. He holds the third rank among the Twelve Kizuki demons (Upper Moon Three / 上弦の参), making him one of the four most powerful demons in the series. Stylistically he reads as a martial-artist demon: bare-chested, tattooed in deep blue patterns, fighting bare-handed with techniques modelled on classical jūjutsu and karate kata rather than the demonic transformations other Upper Moons use.

His defining behavioural tic is the offer he extends to any human swordsman he respects: 'become a demon and fight me again at full strength.' He extends this offer to Rengoku on the train, who refuses. Akaza takes Rengoku's refusal as personally offensive and the fight becomes uncharacteristically vicious; by sunrise Akaza is forced to flee — he physically cannot endure sunlight — and Rengoku is fatally impaled. The scene is famous in Japanese fandom for one image: Rengoku, half-disembowelled and still standing, holding Akaza's hand to the sword wound so that the demon cannot retreat before dawn finishes him. Akaza escapes, but the staging is the moral victory.

Akaza's human past — he was originally a man named 狛治 (Hakuji), a Edo-period jūjutsu student who lost his fiancée and his master to poisoning — is revealed much later in the series (Vol. 18). Knowing this past changes the meaning of his obsession with physical strength: the kata he uses against Rengoku are forms his late master taught him, the speech patterns he uses are pre-Meiji rather than Taishō, and his refusal to fight women is a residue of his fiancée. None of this is visible in the Mugen Train arc itself, but Japanese readers who have read Vol. 18 carry that knowledge backward when re-reading the Rengoku fight.