He is voiced by Toshihiko Seki in the anime.
He is a young man with pale skin and red glowing eyes and he has black curly hair. Muzan wears a white fedora along with a black suit and a white tie. When attacking, he reveals his fangs and his nails grow sharper and turn blue.
Muzan is cold-hearted and cruel, while also showing very narcissistic and arrogant tendencies. He sees himself as perfect, since he is ammortal and very powerful. The only thing, he's weak to, is the sun. That is why he turned other humans into demons and spread his blood, so that one of them might find a solution against the sunlight, so he can become truly perfect. He mostly wants to keep himself out of danger by using his allies and wants to enjoy his life without any harm coming to himself.
He has a very inflated ego and doesn't think highly of anyone but himself. Muzan also doesn't feel attachment to any of his subordinates and discards or sacrifices them, if he feels like it. He is quite merciless when it comes to failures or any sort of disrespect towards him and he doesn't mind killing people in an instant. He forbids his subordinates of revealing his identity to anyone. He also cursed them so that if they say his name, they die a violent death.
Muzan is very cunning and intelligent. He is perceptive enough to be able to seemlessly blend into human society and he even started a family. He can hide his evil side and put on a friendly facade if necessary. He is manipulative and can quickly get people on his side for him to use them. His cruel and imposing nature has gained him great admiration of many demons.
After becoming a demon, he spread his demon blood as much as he could. By giving a select few even more of his blood, he created a group of very powerful demons, called the Twelve Demon Moons, who follow all his orders. Once he was almost killed by an ancestor of the Kamado family. Since they possess the "Breath of Fire" skill, he was at a disadvantage and suffered a crushing defeat. Since then he holds a grudge against the family and wanted revenge.
So one time, he traveled to the mountains where the current Kamado's lived and slaughtered the four children and the mother. However, some of his blood entered the eldest sister, Nezuko, and turned her into a demon. He then retreated to his family he set up and went undercover again.
Eventually, Tanjiro Kamado coincidentally picked up his scent and approchaed Muzan. Muzan wasn't expecting to see him at all and was shocked at first. But since he is walking with his daughter he puts up a kind facade and pretends to wonder who Tanjiro is. Tanjiro is mad at him and wants to kill him. In order to get away and distract Tanjiro, Muzan slices a random passerby's neck and turns him into a demon. While that demon attacks his wife, Muzan gets away with his family. Before entering the carriage, he tells his wife he has to do something and they should go ahead.
He then walks through an alleyway where a drunk guy bumps into him. The drunk starts mocking him and states, he doesn't like fancy-pants like him. When he adds, that he's so pale, he looks like he had died already, Muzan smashes him into the wall, killing him instantly. The muscular brother then angrily attacks him, only to be kicked into the air and be killed. The drunk's girlfriend holds his corpse, crying. He then rhetorically asks her if he looks pale or weak, then replied to himself, stating he is infinitely close to perfection. He then stuck his finger into her forehead. Due to her body receiving so much blood from him, that it couldn't handle the demon transformation, she melted away and died in a gruesome manner. Muzan then calls his subordinates, Yasaba and Susumura, and orders them to kill Tanjiro.
- His design with the white fedora and curly hair, reminded a lot of viewers of Michael Jackson.