Muzan

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Muzan Kibutsuji is the main antagonist of Demon Slayer: Kimetsu no Yaiba. He is the Demon King, the first of his kind, and the progenitor of all other demons in existence. Muzan is also the leader of the Twelve Kizuki, an organization of the twelve strongest demons in existence that serve directly under him.

a millennium ago, during the Heian Era, Muzan was turned into a demon during an experimental treatment by his doctor, who attempted to cure his terminal illness. Making him a vampire who makes demons/aka his minions because he believes they are humans but surperior.

Muzan is a tall and muscular man, with an unnaturally pale skin complexion that can be easily confused as being unhealthy and sickly. He has curly black hair, and sharp, almond-shaped eyes. His irises are cat-like in appearance and colored plum red. He also possesses sharp, blue nails on his hands. He appears to be in his twenties.

Muzan is cold-hearted, ruthless, intimidating, and exceptionally intelligent. He sees no value in any humans he has encountered and shows little value in his own subordinates. Rarely ever visually upset or happy, he appears completely monotone and detached on most occasions, even when he callously murders his own subordinates. The only times he ever shows emotion are when his vision of perfection is moving forward or when he has subordinates completely submit to his will. The manga shows him to be more expressive, particularly when enraged and his facial features are exaggerated to demonstrate as such (possibly as a way to display his normally unreadable emotions). In the anime, his expressions change slightly and his tone of voice tends to be consistent at nearly all times, accentuating his monotonous demeanor. His cruel and imposing nature has gained him the great admiration and fear of many demons.

His inflated ego and delusional sense of self worth are so large that he cannot stand being mocked or corrected in any fashion, believing that others only exist to serve him. It also means he rarely ever takes any opponent or threat seriously. He brutally killed a drunk man simply for being rude to him, and killed the man's brother and a female companion when they had done nothing, all while proclaiming his own perfection and superiority over them, displaying his pettiness as he won't settle for any mockery and will do anything he pleases until the humiliation is remedied. This event also hints that Muzan has unpleasant memories of his weaker, human past, and is very self-conscious of his unnaturally sickly appearance, immediately following with a lengthy denial that he is unhealthy, sick and much less on the verge of death, in all likelihood a bitter reminder that he is still imperfect. When he confronts Kagaya Ubuyashiki, the man responded to Muzan in a tone of pity and sadness for the life he had led. However, Muzan only interpreted it as insults and threats, which further accentuates his hatred for humanity and the Demon Slayers.