Just How Powerful Is Demon Slayer's Nezuko?
After being turned by Muzan Kibutsuji during his slaughter of her family (with the exception of her elder brother, Tanjiro), Nezuko refused to act like other demons. In the "Demon Slayer: Kimetsu no Yaiba" universe, humans are turned into demons, often against their will, and feast on humans to sustain themselves and grow stronger. Often acting on instinct, especially early on in their transformation, most demons can't resist their hunger.
This is just one of many ways that Nezuko has refused to conform to the expectations of a demon. She has never devoured human flesh, and even in her most desperate moments, she refuses to do so. Beyond that, she travels with Tanjiro in a little light-tight box that he carries around like a backpack. Nezuko is a demon who fights on the side of demon slayers. Although she struggles to communicate the way that she once could, she has retained enough of her humanity that even other demon slayers haven't taken her out. Tanjiro couldn't bring himself to kill her because she's his little sister, but other warriors who fight her kind have no such notion. Yet, she lives.
Muzan Kibutsuji, however, is the King of Demons. The demons he feeds his blood to are notably powerful. He is the leader of the Twelve Kizuki, the most powerful demons in existence next to him, and each of them has received a questionable amount of his blood. Since Muzan Kibutsuji is also the person who turned Nezuko, it's safe to say that she is also more powerful than her sweet appearance would suggest.
Nezuko's power starts with her refusal to eat humans
Since demons devour humans to grow more powerful, it could be an easy mistake that Nezuko, despite the demon that turned her, may not actually be all that powerful. After all, she refuses to hurt humans and has sworn to protect them, so where does her strength come from? How does she even sustain herself? The simple answer is that she sleeps a lot. Somehow, Nezuko has managed to replace her need to devour humans with sleep. The more energy she exerts, the longer her slumber, but it actually works for her.
That hasn't stopped her from becoming incredibly powerful, though. Throughout the anime and manga series, fans see her abilities develop and grow. Beginning with simply refusing human blood, even when presented to her, Nezuko proves that she's powerful in mind first. She quickly discovers the ability to change her size as she shrinks down to accommodate the box Tanjiro carries her in during the day. Like most demons, she has regenerative abilities that are dependent on the strength of the individual demon. At first, her regeneration is slow due to her reliance on sleep, but as she grows in power, so do these abilities.
She also develops pyrokinesis, her Blood Demon Art, which manifests in pink flames that can actually disrupt the Blood Demon Arts of other demons. It gives her a powerful advantage in battle, but she has also been able to use her pyrokinesis to cure Tanjiro and her other friends after being poisoned. In fact, her Blood Demon Art can't actually harm humans, but it can stop a demon dead in its tracks.
Nezuko even outshines Muzan Kibutsuji
Although the anime and the manga are at two separate parts of the story, and the manga has advanced far beyond the anime, it should come as no surprise to the anime fans that Nezuko's power continues to rise. Eventually becoming powerful enough to take on Upper Rank demons, Nezuko even gains an "awakened" demonic form. Taking on an adult form, Nezuko develops a single horn protruding from the right side of her head and what looks like vines or protruding veins across most of her body. In this form, her abilities are far more potent.
In Season 2, Episode 6, Nezuko takes on Upper Rank 6 Daki, where she first unlocks her awakened form. Her first transformation doesn't go smoothly. Losing herself to her feral nature, she actually does attack a human, but Tanjiro intervenes before she can do any harm. As her power continues to grow, she eventually learns to control this form as well, using it as she pleases. At this point in her growth, she's able to go toe-to-toe with Upper Rank demons and could easily be classified as one herself.
As the manga advances further, it's revealed that Nezuko can do something else that no other demon has ever been able to achieve — she can walk in the sun. Even Muzan Kibutsuji has been unable to obtain this power. It's also worth noting that the Demon King can't actually control Nezuko. As the King of Demons, he has the ability to control every demon as he pleases, but not Nezuko. Not only has Nezuko evaded his influence, but in the later manga arcs, she is also actually surpassing him in some aspects.