Kibutsuji Muzan

The main antagonist of this work, Muzan Kibutsuji, is the first demon born over a thousand years ago. He is the absolute ruler of the demons, having created a multitude of them by giving his blood to humans. He killed Tanjirou Kamado's family and turned Nezuko Kamado into a demon.

He controls the demons with a curse embedded in his blood, known and feared as "that person." He can freely change his appearance and attacks, and is capable of killing immortal demons. His personality is cold, merciless, and domineering; he never forgives those who do not follow his will, and he has brutally slaughtered the lower-ranked demons who were loyal to him over trivial matters, and has unreasonably reprimanded Akaza when he came to report. Tamayo sarcastically describes him as "always a coward scared of something." He often acts violently in a fit of rage and is hypocritical in his actions.

His true identity was a noble born during the Heian period, an ancestor of the Ubuyashiki family. He was weak and expected to die by the age of twenty, so a doctor prescribed a new medicine hoping for recovery, but when his condition did not improve, he killed the doctor in a fit of rage. Ironically, the medicine began to work shortly after, giving him a robust body but making him unable to go out in the sunlight. Thus, he became the first man-eating demon craving human flesh.

His goal is to overcome sunlight and achieve true immortality, and for this, he has two plans: "create a demon that has overcome the sun and absorb it" and "search for the blue spider lily, the raw material for the medicine that turned him into a demon." He continues to create demons solely for his own use, and he refers to other demons, including the upper ranks, as "unwanted kin."

He is elusive and mysterious, and even the Hashira had not made contact with him until Tanjirou Kamado tracked him down by scent in Asakusa, revealing that he was blending into human society. He uses various appearances and identities, such as the businessman "Tsukihiko" who runs a trading company with his wife Rei and daughter, the adopted child of a wealthy family "Toshikuni," and a female geisha used during the search for the blue spider lily. He disguises his inability to walk under the sun as an illness.

He has a deep grudge against Yoriichi Tsukiguni, the "swordsman with the sun earrings." He is particularly wary of Tanjirou, who wears the same earrings, and Tamayo and Nezuko Kamado, who have escaped his control, marking them for elimination.

His visible human-like neck is not his vital spot; even if his head is smashed, he can regenerate, and his body contains five brains and seven hearts as spares, which also move positions and cannot be targeted. In combat, he transforms into a grotesque form with multiple mouths and fangs all over his body, turning body slams into predatory attacks. He also swings countless tentacles at high speed to slash at enemies around him. His strength is such that he can overpower nine people, including five Hashira, at once. However, he was no match for Yoriichi and avoided a rematch until Yoriichi's death, after which he and Kokushibo massacred all those related to the Sun Breathing technique.

After receiving the self-destruction of Yoriichi, the humanizing drug from Tamayo, and a decisive blow to the neck from Himejima, he fell into the Demon Slayer Corps' trap in the Infinite Castle and awaited recovery in a flesh cocoon. His hair turned white, and his body, mostly covered in a mass of mouths, declared the annihilation of the Demon Slayer Corps.

After a fierce battle with the Demon Slayer Corps, he was finally defeated by the morning sun. Moments before his destruction, he misinterpreted Yoriichi's words "Feelings are immortal and eternal," was moved to tears, and tried to entrust his will to destroy the demon slayers to Tanjirou by pouring his blood and power into him, turning him into the "King of Demons." However, with the efforts of Nezuko and the Demon Slayer Corps, Tanjirou nearly returned to human, and Muzan's attempt to hold him back failed, resulting in Muzan falling into the deepest layer of hell, Avici.