While she might not be as powerful as Muzan himself, her overall strength makes her easily comparable to an Upper rank demon.
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.
Since Nezuko isn't a part of the Twelve Kizuki, she doesn't get a demon rank. She does however have her own demon blood art, as well super-fast reflexes and the ability to grow and shrink in size.
However, the trait that sets her apart from all other demons is the fact that she has the ability to conquer the sun, prompting the Demon King to call her the "chosen demon". Abnormal Demon Physiology: As the "chosen demon", Nezuko has several unique demonic traits that set her apart from all other demons.
Nezuko doesn't stand a chance against the Upper Moon 3
Nezuko is undoubtedly a strong demon in Demon Slayer and she proved her strength repeatedly. One situation that really displayed her true potential was the fight against Daki. She underwent a transformation and thoroughly overwhelmed the Upper Moon 6 demon.
Muzan, the King of Demons, is the strongest and also the primal demon in the world of Demon Slayer. Consuming his blood is enough to turn you into a powerful demon. He can control all the demons in existence, with only people with strong willpower like Nezuko being an exception.
1 Muzan (Demon Slayer)
Without that antidote, it would have been impossible to kill him. So despite all her strength and humanity, Nezuko would have easily lost to Muzan.
Nezuko's Blood Demon Art is “Pyrokinesis,” which allows her to generate and manipulate special demonic flames created from her blood. It may be because of her unique nature, but her flames have a special ability, making them only harmful to demons and anything related to demons.
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.
Nezuko's Sunlight Immunity in Demon Slayer is Tied to the Blue Spider Lily. Nezuko having eaten the Blue Spider Lily at some point could explain her immunity to sunlight, and Demon Slayer: Kimetsu no Yaiba has already worked to justify that.
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.
Gyomei Himejima is the strongest Hashira in Demon Slayer. Even the Hashiras who are filled with pride, acknowledged his skill and strength. As a child he was able to save a child from a demon by physically beating it for hours. He, along with Sanemi Shinazugawa were able to beat the Upper Moon 1, Kokushibo.
Tanjiro's Current Demon Slayer Rank Is Kanoe
Not many of the Demon Slayers' ranks are known as of Season 2, but there are a few. Tanjiro, Zenitsu, and Inosuke have moved from Mizunoto up to Kanoe due to their exploits. Murata, the Demon Slayer, who was almost digested on Mount Nagumo, is also a Kanoe.
Or: Kamado Tanjiro unwillingly becomes Upper Moon Zero, and he willingly kills for Kibutsuji Muzan, because his sister unconditionally believes that the man saved them (and didn't kill their family), and thinks that they must protect him, and if Tanjiro disobeys, then his sister will be killed.
With the emergence of demons in the modern age, Nezuko returned to active duty as the Sun Hashira and reassembled the Demon Slayer Corps, assuming the leadership position and moving Kiriya Ubuyashiki to the Ubuyashiki-Kamado complex, in order to keep him safe from harm.
Given that she survived the Final Selection, she has excellent combat abilities. But Nezuko single-handedly overwhelmed Daki, showing just how strong she is as a demon.
1 Exploding Blood/Pyrokinesis Blood Demon Art
By far, this is Nezuko's most damaging technique when it comes to fighting her own kind.
So if Nezuko ate a human. and continued to do so over time, her power could rival that of Upper Moon one or 2, and here's why. One, if you're a manga reader, you know, in her base, Nezuko conquers the sun. And 2, her blood demon art can nullify.
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.
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.
Weaknesses: Wisteria flowers are poisonous to her. Exposure to sunlight used to be lethal, but no longer affects her. Notable Attacks/Techniques: Blood Demon Art: Blood Burst: Nezuko ignites her blood and turns them into flames that only burn demons and their blood demon arts.
Giyu is one of the legendary Hashira, and he's brought down demons far more powerful than Nezuko. With his level of skill and impressive agility, he'd easily defeat Tanjiro's little sister.
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 isn't like other demons, which is how she was given the name 'The Chosen Demon'. The reason behind this is because unlike other demons, she can actually walk in the sunlight without getting burned. It is shown in the manga chapter 130, She has walked in the sun.