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”.
It is theorized that Nezuko was able to conquer the Sun because she comes from a lineage of Sun Breathing users and because she kept a big part of her own humanity i.e. because she never ate human flesh.
Sunlight Resistance: During their battle against Upper Rank Four, Hantengu, Nezuko developed immunity from sunlight, a feat thought impossible for demons, even by the Demon King himself.
Nezuko apparently can use Sun Breathing to a certain extent as a member of the Kamado family, since she eventually becomes immune to sunlight.
Sunlight: Muzan is noted to be immune to being killed via decapitation, as seen when Gyōmei destroyed his head and he still managed to regrow it. Thus the only other known way that he can be destroyed is by sunlight.
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 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.
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.
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.
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.
The Wind Hashira is one of the strongest Hashiras in the Demon Slayer series. He played a significant role in defeating Kokushibo and was able to inflict damage on Muzan. Sanemi's sheer power and speed will be able to overwhelm Nezuko even in her final form.
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.
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.
Noting to himself that if he had not been interrupted by the sun, Tanjiro would have surely killed the Kakushi, Giyu wistfully begs in his mind for Tanjiro to stay who he was and die as a human. Tanjiro becomes immune to the sun.
Nezuko's most significant trait, however, is her immense growth, which is likely due to a combination of the amount of blood she received from Muzan and her willpower. To conclude, Nezuko is a formidable powerhouse within the anime and is probably one of the strongest main characters in the series.
The number one ranked Kizuki is the terrifying Kokushibo, whose six orange eyes mark him out as one of Demon Slayer's most striking character designs. He also bears a major resemblance to Tanjiro thanks to his red-tipped hair and flame-marked face, although that appears to be a coincidence.
Nezuko would practically be unstoppable in the series for the following reasons: Nezuko has conquered the sun, meaning she can eat humans both daytime and nightime. Meanwhile, other demons can only eat humans at night. After immunity to the sun, Nezuko would regain her intelligence and speech when she was human.
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.
Nezuko, the young sister of the main protagonist Tanjiro, has a special Blood Demon Art called Exploding Blood that allows her to ignite her blood into pink flames and even summon it from thin air. Unlike other demons, Nezuko doesn't consume humans to grow stronger but instead rests to unlock her full potential.
While stuck physically at age 12 because of the whole demon thing, Nezuko is 14 chronologically. Can Nezuko change her age? 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.
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.
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.
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.