Yet an official
Originally a human, Muzan suffered from a terminal illness, during an experimental treatment, Muzan became the first ever demon. He is also the progenitor of most other demons and the leader of the Twelve Kizuki. He is also the archenemy of the Kamado family, particularly both Tanjiro Kamado and Nezuko Kamado.
The only Demon he despises the most is the Upper Rank Two: Doma.
The warrior Muzan feared was a swordsman named Yoriichi Tsugikuni. Yoriichi was the only slayer before Tanjiro who came close to killing Muzan, and the flashback of their fight occurred in Chapter 187.
However, in truth, as revealed after her death, Muzan sees Daki as a tool to keep her brother, Gyutaro, in line, deeming her to be the main reason why the "true" Upper Rank 6 was killed and in truth does not think highly of her and deeming her as someone not very intelligent.
It is revealed that Muzan's true fear of dying and immense desire to live and survive was due to him being born "dead," where his heart would constantly stop in his mother's womb and was assumed stillborn until he fought to breathe before his cremation.
Doma is always smiling and acts very friendly, but this cheerful, caring nature is simply a facade. He states that he cares for his fellow Upper Moons and even cries when he learns that Akaza died, though, in fact, he doesn't feel anything for them. Even when he was human, Doma didn't feel any type of human emotion.
Anne was a kind and loving soul and was described as the only person that Muzan had truly loved, besides his three daughters.
1 Yoriichi Tsugikuni (Demon Slayer)
Yoriichi's mastery of the Sun Breathing technique, combined with his incredible speed and agility, made him a formidable opponent for Muzan.
Does Muzan have a favorite demon? Despite the fact that Muzan. is always shown to have disdain for his creations, Muzan does in fact have a favorite. On that list is Naki, made due to a convenient blood demon.
The two known demons who broke free of Muzan's curse are Tamayo and Nezuko. Tamayo was able to break free of the condemnation when Yoriichi fought Muzan, nearly killing him in the process.
Who does Muzan regret killing? He shows his most human side to fans, wherein he admits to regretting murdering his doctor. He comments that had he been patient, the doctor would have cured him, and he would never have turned into a demon.
How Doma became the worst demon in Demon Slayer. Almost every creature possesses emotions, be it anguish, sorrow, happiness, or even fear. However, in the case of Doma, he doesn't feel a thing. During his childhood, his mother stabbed his father to death for cheating on her and, later on, took her own life.
After he neutralizes Tamayo's poison and emerges from his flesh cocoon, Muzan's hair grows in length and turns white due to Shinobu's poison accelerating his age. He also grows numerous mouths all over his limbs, each one lined with a set of incredibly sharp fangs.
1) Yoriichi Tsugikuni
His powers were beyond comparison as he defeated Muzan once and had him on the verge of death until he managed to escape. Even in his eighties, he was still in his prime as he slit the throat of his twin brother Michikatsu Tsugikuni, who is turned into a demon and goes by the name Kokushibo.
Akaza was also one of Muzan's favorite demons out of all his subordinates. The reason behind this is Akaza's extreme loyalty to his master. The Upper Rank Three demon was always sent away on special missions, something the Upper Rank 1 and 2 were often denied of.
1 Muzan (Demon Slayer)
Without that antidote, it would have been impossible to kill him. So despite all her strength and humanity, Nezuko would have easily lost to Muzan.
Kokushibo is one of the strongest demons in the anime, only second to Demon King Muzan. Before turning into a demon, he was a part of the Demon Slayer Corps alongside his brother Yoriichi Tsugikini.
The strongest demon to exist, Kibutsuji Muzan, is the main antagonist of the show. He is arguably one of the strongest characters in Demon Slayer. Based on his strength and abilities, it is doubtful that anyone can individually take on Muzan. But he was eventually conquered, albeit not by any one individual.
Over his long lifetime, Muzan had a number of families. This includes the one he was born into before becoming a demon. Additionally, he took 5 wives, making each one's life miserable.
Thus, Muzan created demons for hundreds of years even when he didn't want to turn someone. Since he didn't know anything about the Blue Spider Lily medicine, he needed others to find the plant for him by happenstance. This is the general reason why Muzan turned Nezuko into a demon.
For those who have watched only the first season of the anime, they will be surprised to know that Muzan turns into a woman in the second season. He keeps transforming in order to hide his true identity, and he's even known to change into an 11-year-old child to hide from Demon slayers.
Akaza. Because of his desire to become stronger and Doma's strength allowing him to surpass himself to become Upper Two, Akaza harboured an intense hatred of Doma. Doma, as apathetic as he is, considered him his "best friend", seemingly ignoring the contempt Akaza possessed.
However, after his death, he accepted that he is wrong and that Heaven and Hell actually do exist. Doma also displayed what arguably is his first true emotion, love, when he proclaimed his love for Shinobu after finding her cute.
Douma said that he had never felt emotions before, but his heart felt like it was beating now. He started doubting his belief that heaven and hell didn't exist and started blushing at Shinobu, quickly developing a crush on her. He told her that she was cute and asked if she wanted to go to hell with him.