History. Muzan was born in Japan's Heian era, to a constant struggle to survive on account of his frailty. His heart stopped multiple times in his mother's womb, and when he first entered the world, he was deemed stillborn, having no pulse and not breathing.
Muzan was diagnosed with a very rare unnamed illness and should have died at the age of 20. His doctor then traveled all around Japan and found the mystical Blue Spider Lily. Using it as part of his medicine that seemingly cured Muzan, but the doctor's cure was incomplete and Muzan still suffered occasional relapses.
Muzan's Blood Demon Art is known as “Biokinesis.” It allows him to alter his body and shapeshift at will. Although all demons have this innate ability, Muzan elevates it by modifying his flesh and organs. Biokinesis is also the only reason Muzan has been able to hide from the Demon Slayer Corps for 1000 years.
Muzan's abilities as a demon begin with how his body is composed. With the power to turn his body into anything he desires, his appearance can change, and he can move his internal organs around really quickly. He took the form of an infant and that of a woman to achieve some of his aims.
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.
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.
Based on the familiarity in which the two conversed at the end of the story, and the fact that she was beside him when he fought Yoriichi, suggest that he did have some fondness for Tamayo, though why in never covered.
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.
2 His Body Is Weak To Certain Poisons
Poisons that did not exist before, like Tamayo's concoction of turning demons into humans, are the kind of poisons that Muzan's body isn't prepared for, as he's never been exposed to them before, and thus, his body has no idea as to how it should fight those poisons.
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.
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.
Muzan's loyal wife and daughter seem like convenient test subjects to help him synthesize a cure for sunlight weakness as a backup in case the Blue Spider Lily is never found. The young girl might even be Muzan's biological child, a half-demon who gives off the scent of a human while having his essence in her.
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.
1 Muzan Regretted Killing His Doctor
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.
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.
He was plagued by health issues even in his mother's womb, and was stillborn as a result, but Muzan fought for life and began crying for the first time as his family was about to cremate his body.
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.
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.
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. nxt. Hanafuda Earrings are a symbol of Sun Breathing users, traditionally passed down from generation to generation as a family heirloom.
Tanjiro's ancestor was a good friend of Yoriichi and arguably was one of his only true friends. Therefore Yoriichi gave the earrings as a gift, and since then they have been passed down the generations until they reached Tanjiro.
He cannot even begin to comprehend how they fight with no regards to their own lives. Muzan is someone who lives in constant fear of his own mortality. Despite being the most powerful being on earth, he's always afraid, he's always hiding himself, and he's always alone. He's a coward ultimately.
Tsukito, Yorogi, Kuraku, and Isatsu all agree that they are loyal to Muzan.
Haruka Shiraishi: Muzan's Daughter.
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.