Akaza

Upper Moon Three. A young demon with patterned tattoos all over his body and short crimson hair. The first Upper Moon to appear in the story. Muzan favors him for his loyalty and seriousness. In the Infinite Castle, he is reticent, but becomes talkative when dealing with people. He has stated that he likes talking to humans and calls those he acknowledges by their first names. According to the fanbook, perhaps a remnant of his human days, he unusually spends more time training than eating humans.

He has a straightforward martial artist-like side that continuously seeks strength. He respects powerful enemies but openly despises those he deems weak, often trying to kill them for trivial reasons. He trained for hundreds of years while young and strong, praising becoming a strong demon like himself as "the supreme realm," and despising humans for weakening with age. He respects those with high abilities and invites them to become demons.

Known as "Fist Demon," his combat style involves physical fighting. He also uses a Blood Demon Art that detects fighting spirit.

As a human, he was born in the Edo period under the name Hakujii. To pay for his sick father's medicine, he pickpocketed, but his father, distressed by this, committed suicide, and Hakujii was banished from Edo. After being picked up by Keizo, who ran a martial arts dojo specializing in unarmed combat, he fell in love with Keizo's daughter, Koyuki, and vowed to become stronger than anyone to protect her. However, after Keizo and Koyuki were poisoned by a neighboring swordsmanship dojo, he massacred 67 people from that dojo in a rage. Desperate, he was turned into a demon by Muzan and lost his memories, becoming a ghost seeking only strength. Techniques reminiscent of his human days, such as "Snow Formation" and "Fireworks," are based on his memories. Even as a demon, his feelings for Koyuki remain subconsciously, and unlike Doma, he does not consume women. He despises Doma, who tries to be friendly unilaterally. Both "Hakujii" and "Akaza" are names derived from guardian dogs, and the episode that delves into his past is titled "The Useless Guardian Dog."

After Yume was defeated, he appeared under Muzan's orders. He acknowledged Rengoku's strength and invited him to become a demon, but they clashed. Although he inflicted a fatal wound, his movements were sealed. Panicking as the sunlight began to rise, he attempted to flee. Just then, Tanjiro's Nichirin Sword pierced him, and he retreated with the sword still in him, being called a coward by Tanjiro. When reporting the mission's outcome to Muzan, he was reprimanded for failing to kill the demon slayers present and was punished. This led him to target Tanjiro.

In the Infinite Castle arc, he fought against Tanjiro and Giyu, two against one. Even with Giyu's awakened mark and Tanjiro's near-pillar strength, Akaza surpassed them, but he could not detect Tanjiro's "Transparent World" fighting spirit and was decapitated. However, as a demon, he tried to regenerate his neck, but his human memories resurfaced, realizing he had lost the reason to become stronger and that the weak person he hated enough to kill was himself, who couldn't protect his loved ones. Accepting death, he attacked himself and crumbled away. After death, he reunited with his father, Keizo, and Koyuki, tearfully apologizing to Koyuki for not being able to protect her and joining her in the flames of hell.