She might not be a part of the 12 Demon Moons, but she has the potential to overpower an Upper-Rank demon with ease. Unlike other demons who survive on blood, Nezuko never really needs it, and she doesn't even consume human food either.
Nezuko refusing to drink Sanemi's marechi blood. She is awakened by Sanemi, who stabs her in an attempt to provoke her and her brother. Sanemi trying to prove that Nezuko is dangerous, uses his marechi blood as a bait. Nezuko recalls her family and refuses to drink it at the end.
Muzan Kibutsuji may have tried to kill Nezuko by exploding her after injecting a lot of his blood into her veins. But she was somehow able to survive with that much of Muzan's blood, that's what possibly made her a powerful demon.
Most notably, Nezuko can continuously restore her power without consuming any human blood like any other demon would need to do. Instead, she recovers her strength through sleep alone, though she would have to spend a lot of time sleeping in order to recuperate and regain her power.
This was also almost completely true for Nezuko. After she transformed, she was so hungry and had no control when she attacked her own brother in an attempt to eat him. However, she was subdued before ever tasting human blood.
The now-iconic bamboo muzzle serves as a way of stopping Nezuko from using her teeth, and also keeps her from tasting human blood, which would cause her to lose control of herself.
According to Doki, nezuko's already more powerful than Upper Moon. 6 in her awaken form. 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.
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.
His search came to an end when he discovered that Zuko was able to withstand the sun. While this is a great plot point, fans are confused as to how a demon whose main weakness is the sun can survive it. This is why Kamado Nezuko was able to stand in the sun despite being a demon.
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.
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.
Since Nezuko is from a Sun Breathing lineage she can survive in daylight. After the final battle against Hantengu concludes she can step into daylight and many theories are available about the phenomenon. One of the theories is that her abstinence from consuming human blood is what saved her.
While she might not be as powerful as Muzan himself, her overall strength makes her easily comparable to an Upper rank demon.
Nezuko's Blood Manipulation is another special ability
In her awakened form, she has shown immense control of her blood. Nezuko can even control her severed body parts at will.
He tells him that he will allow Tanjiro and Nezuko to join the Corps. But five of the Hashira are unable to accept this decision right away. The wind Hashira, Sanemi Shinazugawa, slashes his own arm and taunts Nezuko with his blood, trying to get her show her true colors as a demon.
One of Nezuko's most remarkable abilities is that she can solidify her blood to reassemble her dismembered body, possibly even to the extent that she could survive decapitation.
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. Hence, Muzan was searching for the Blue Spider Lilly, a flower that was a crucial ingredient in the medicine used by the doctor who saved his life.
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.
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.
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.
However, in later volumes, the muzzle is removed and she gains the power of speech in volume 22 then remembers everything and gets her human thoughts again and remembers her name, albeit very broken due to not speaking for more than two years.
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.
Secondly, Muzan would probably… eat Nezuko and Tanjiro in the hopes of obtaining their sun-resistance abilities, and also since he wouldn't be able to convince them to become demons to work under him. This would allow him to become the ultimate demon.
"Humans are to be protected and saved... I will never hurt them." - Nezuko Kamado.
In the absence of Nezuko, Tanjiro would have been unable to find it in himself to sympathize with the fear and sadness of demons as they died. For example, Kyogai, the drum demon, trapped people in his labyrinth-like house to kill and consume them.