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.
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 and both Tanjiro Kamado and Nezuko Kamado.
In the final battle, even working together, the demon slayers and Hashira cannot defeat Muzan. He is simply too powerful. But the toxin created by Tamayo, Yushiro, and Shinobu slow him and weaken him considerably so that the demon slayers are able to hold him off until sunrise.
Muzan Kibutsuji is by far the worst of demonkind in Demon Slayer, objectively or otherwise. Everyone, including his most loyal subordinates, is terrified of him, especially in light of his fickle impulses. That said, his unpredictable nature wouldn't have been a problem if he hadn't also been such a malicious monster.
The demon slayers successfully defeated Muzan.
The only Demon he despises the most is the Upper Rank Two: Doma. It is quite a shocker that the second most potent Demon in the Demon Slayer is not Muzan's favorite.
Yoriichi Tsugikuni, the first swordsmen to ever use Breathing Styles, once nearly killed Muzan with Sun-Breathing, and Muzan has been both physically and psychologically scarred ever since.
Tsukito, Yorogi, Kuraku, and Isatsu all agree that they are loyal to Muzan.
Abilities. Demon Body: Unique from other demons, he possesses seven hearts, and five brains. These are responsible for his powerful vitality, should any of them be harmed, he will be severely weakened.
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.
They've always existed as villains, notably with people like Starscream in Transformers. The difference here is that Muzan is the big bad, someone who's meant to be the alpha and omega in the series. Your top tier villain is rarely cowardly. It's what makes Muzan so interesting as a character.
Moreover, Tanjiro's appearance somewhat resembled Tsugikuni. So the only reason Muzan is afraid of Tanjiro is because of the Hanafuda earrings he possesses, which acted as a reminder of his downfall.
14) Muzan Kibutsuji (over 1000)
Being a demon, Muzan is capable of living for many, many years. However, unlike other demons that have aged over hundreds of years. Muzan is well over 1000 years old.
Trivia. Anne was originally planned to be Muzan`s older sister, who died by the hand of a Demon Slayer but that idea was scrapped, as the author knew practically nothing about Muzan`s early life. Instead, Anne became his wife, by which he had two children.
Muzan himself never thought highly of Daki and didn't reciprocate her devotion whatsoever, only ever talking to her to sweetly manipulate her into defeating Demon Slayers on his behalf.
Despite being a blood-thristy demon who has murdered several people, he has a set of principles he adheres to. One thing that Demon Slayer fans have noticed is that Akaza refuses to eat or kill women. Additionally, the Demon King, Muzan, gave Akaza special permission to do so.
However, Muzan would never fully escape his intimate relationship with death, as he was later diagnosed with a terminal illness that would take his life before he reached his 20th birthday.
Some of the characters who have faced Yoriichi and survived
This happened due to a treatment that had gone wrong. While he was being treated, Muzan killed the doctor because he was not able to observe any results.
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. More specifically though, was the fact that Nezuko was of Kamado blood.
The young Tanjiro is not only one of the greatest demon slayers, but thanks to his kind nature, he's also one of the most honorable characters of the series. Tanjiro not only showed selflessness and love towards his sister and the people he cared for but even showed kindness towards his demonic enemies.
Demon Slayer's adorable icon Nezuko has stolen fans' hearts for five key reasons, earning her the title “Best Girl” of the series.
Since she's the Love Hashira, it just makes sense for Mitsuri to be one of Demon Slayer's kindest and most compassionate characters. She always compliments the people around her, even if it's just mentally. She can also find everybody's best qualities.