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.
It was shown that Nezuko conquered the sun, making her immune to sunlight.
The Kamado family has a connection to Sun Breathing, the original and the strongest breathing technique in Demon Slayer. While Nezuko cannot use this breathing style, her ancestry may have influenced her fire-themed Blood Demon Art and allowed her to stand out from other demons.
Muzan injects Tanjiro with all of his blood, turning him into a demon. Tanjiro begins to become immune to sunlight.
Nezuko did indeed conquer the sun due to a number of reasons: Tanjiro bringing back her human self. Urokodaki's hypnosis. Her connection with the sun breathing techniques.
Apart from Tanjiro, the show centers on his younger sister Nezuko Kamado, turned into a demon by Muzan Kibutsuji. Demons are incapable of walking out in broad daylight because the sunlight will burn them into the ashes. However, Nezuko has immunity against the sun, so she was given the name “The Chosen Demon”.
In episode 6 of Demon Slayer season 2, Nezuko entered a berserk state to protect her brother Tanjiro, causing her body to develop into that of an adult. She loses her gag, grows a single horn and gains vine-like patterns across her body.
Abnormal Demon Physiology: As the "chosen demon", Nezuko has several unique demonic traits that set her apart from all other demons. Most notably, Nezuko can continuously restore her power without consuming any human blood like any other demon would need to do.
8 Tanjiro Is The Only Living User
On the other hand, Sun Breathing can, presently, only be performed by Tanjiro Kamado. Though its techniques can be taught to others provided they have the guidance of a Kamado, there are no other known users who are currently alive.
With this in mind, if Nezuko had also been murdered by Muzan, Tanjiro would have no reason to be empathetic toward demons. Her transformation gave him a perspective of both demons and humans, but without Nezuko becoming a demon he would have no reason to take their feelings into account.
Nezuko is at least stronger than Daki, meaning that if Muzan could bring her back under his control, she would certainly qualify for the most prestigious title a demon can hold: a member of the Twelve Kizuki. Thanks to Nezuko's remarkable strength, she would have to be an Upper-Moon.
"Humans are to be protected and saved... I will never hurt them." - Nezuko Kamado.
Nezuko has several powers, including regeneration, superhuman strength, growing and shrinking rapidly and a blood demon technique called Exploding Blood (爆血, Bakketsu) that causes her blood to burn once it leaves her body.
Unlike other demons who survive on blood, Nezuko never really needs it, and she doesn't even consume human food either. Moreover, Nezuko has never shown any signs of undernourishment as she is always ready to assist her brother with her overwhelming strength and deadly Blood Demon Art.
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.
"Breath of the Sun: Thirteenth Style") is the thirteenth form the Sun Breathing and the pinnacle of all Breathing techniques in history. This technique was created by Yoriichi Tsugikuni himself in order defeat Muzan Kibutsuji.
1 Yoriichi Tsugikuni
Yoriichi was the very first user of the Breathing Styles and he came up with Sun Breathing, a powerful breathing form that is known to be the most effective against demons.
The muzzle serves as a representation of Buddhist practices. First, it gags Nezuko, preventing her from speaking like a Buddhist monk's vow of silence, but it also is there to suppress her desire to eat humans. Through a Buddhist lens, Nezuko's muzzle is the symbol of a Buddhist monk.
Tanjiro is ready to do anything, even die, if it means saving his sister. It becomes clear that Tanjiro's only choice is to join the Demon Slayer Corps himself and despite the intense, dangerous training, he doesn't hesitate. He loves Nezuko more than he fears demons or even death.
Moreover, while she was asleep, Urokodaki performed a unique hypnotism on Nezuko, making her believe that all humans are her family and she must not hurt them in any way. With her immense fondness for humanity, Nezuko broke out of Muzan's control, becoming free to do anything at her will.
Due to her demon abilities, Nezuko can change her size/age and appear “mature” in a slightly older form or can even make herself look younger and smaller than a 12-year-old. Anime is weird and demon stuff only makes it weirder.
Nezuko's Blood Demon Art, called Exploding Blood, allows her to ignite her blood into pink flames or even conjure it from thin air.
Nezuko's most distinguishable physical trait by far is her bamboo muzzle. Given to her by Giyu following their first, nearly fatal, encounter, Nezuko is rarely seen without it.