During his final struggle, and as a way to slow down the effects of the sun, Muzan adopts an enlarged demonic baby form, approximately as tall as a 3 story building when standing up. In this form he has very pronounced blood vessels across his body, small claws in his hands and fanged teeth in his mouth.
Initially, Muzan had considered Kyogai as a special Demon with an invaluable power due to his Blood Demon Arts, earning Muzan's favor and causing him to have high hopes for Kyogai's continued development as a demon as he grew stronger.
Muzan is an extremely self-absorbed and narcissistic demon. He is obsessed with becoming the world's most superior being, and, since he's lived on for centuries without any real threat to his life, he has let that go to his head. Aside from the sun, he has no real weakness, and he obliterates whoever gets in his way.
Shapeshifting: Muzan can control his body's shape and even his presence, making him able to transform into someone completely different, without even those with extrasensory perception noticing, though he cannot fool Tanjiro Kamado's sense of scent.
At some point in her life, Anne was given to Muzan Kibutsuji as a sacrificial maiden (so that her entire family would be spared); the couple had two children together (during which she died after giving birth), and adopted a half-human/half-demon child from the streets, naming her Sakura, the name given to their second ...
Roughly 1,000 years ago, during Japan's Heian Period, Muzan Kibutsuji was declared stillborn after his heart stopped beating multiple times inside his mother's womb. As he was about to be cremated, Muzan coughed and squirmed on the pyre, indicating this young, sick infant still had the will to live.
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.
Origin. The first demon that is said to have existed was Muzan Kibutsuji. The one who turned him into a demon was a generous doctor from the Heian Period, who wanted to save Muzan from death since, at the time, he was diagnosed with a disease which would kill him before he turned twenty.
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.
However, she wasn't immune to the sunlight initially, as she got burned badly at first. So, in conclusion, her coming from a Sun Breathing user's lineage and not relying on human substance are perhaps the reasons for her immunity.
There is no logical reason in relation to the plot that suggests why she has a horn. While fans might assume the horn grows on people with some relation to the sun, this wasn't the case for Tanjiro, who also turned into a demon. The only other reason that could explain the horn's existence is purely aesthetic.
The only reason why Muzan hates Doma is because of his insincerity and lack of emotions. This might sound odd, expecting emotions from a demon, but even a demon has to retain such tendencies to keep things in order.
This willpower is also what helps Nezuko truly utilize her Awakened form, to fight more efficiently. While she might not be as powerful as Muzan himself, her overall strength makes her easily comparable to an Upper rank demon.
The answer to whether Upper Moon One is stronger than Muzan in Demon Slayer is quite clear; Kokushibo is not stronger than Muzan Kibutsuji. The main piece of evidence here is that Muzan was able to escape a living Yoriichi, whereas Kokushibo needed death's assistance to prevail.
Unlike other demons that aged over hundreds of years, Muzan is well over 1000 years old. Although he is old, Muzan has the appearance of an adult man in his 30s. The reason being he drinks a medicine that keeps him young and immune to illness.
Muzan is searching for the blue spider lily to gain immunity against the sun so he can perfect immortality. In chapter 127 of the manga, Muzan's backstory reveals that he needs the blue spider lily to conquer his weakness, or a demon who could resist the sun so he can absorb their power.
Muzan has been looking for centuries of a demon that can conquer the sunlight. After Muzan discovers Nezuko's ability, he decides to change his plans and pursue her for himself. He believes that by devouring her, Muzan can also be able to walk freely into the sunlight.
Just as previous “Kimetsu no Yaiba” manga spoilers had predicted, Muzan uses another transformation to deal with the situation. His flesh grows outward and into a giant baby as the demon plans to use the added layers of flesh as his shield against the sun.