WARNING: The following contains spoilers from the Demon Slayer: Kimetsu no Yaiba manga by Koyoharu Gotouge, available in English through VIZ Media.
Nezuko Kamado is Demon Slayer: Kimetsu no Yaiba's biggest wild card, a demon who fights alongside Demon Slayers. After being transformed into a demon by Muzan Kibutsuji, the Progenitor of Demons, Nezuko changes the game by refusing to consume human flesh like others of her kind do. As her brother Tanjiro trains as a Demon Slayer in hopes of defeating a demon with powerful enough blood for a cure to be developed, Nezuko has begun fighting at his side, using her demonic powers for good instead of evil.
An utterly unique demon, Nezuko is not only able to resist the temptation of human flesh, she can survive and even thrive without it. In the story so far, it has been several years since Nezuko became a demon and she has never once harmed a human. During that time, her strength has increased as dramatically as Tanjiro's until she is able to take on Upper-Rank demons, the most powerful in the world.
Like all demons, Nezuko has a special ability called a Blood Demon Art. Hers is referred to as Pyrokinesis, with which she conjures bright, pink flames. Nezuko's ability is devastating for other demons, as her flames have the power to cancel out other Blood Demon Arts. Thus far in Demon Slayer, she's used her flames to weaken Lower-Rank Five Rui's webs, break Tanjiro and Inosuke out of Lower-Rank One Enmu's dreams, and even cured Tanjiro, Inosuke and and Tengen Uzui from Upper-Rank Six Gyutaro's deadly poison.
In the "Entertainment District" arc, Nezuko battles Daki, a demon who shares the title of Upper-Rank Six with Gyutaro. During this bout, Nezuko unlocks a new, more demonic-looking form that transforms her into an adult and sends her power through the roof. In this state, Daki admits that Nezuko's power and regenerative abilities are on par with an Upper-Rank. The fact that demons are incapable of killing one another is the only reason Daki survives her fight with Nezuko in this form.
Unfortunately, the tradeoff of this power is that it causes everything about Nezuko to become wholly demonic. This form's ferocity causes her to attack a human, but fortunately her brother is able to stop her before she hurts anyone. In the following "Swordsmith Village" arc, Nezuko exhibits greater control over this form, using it to battle Upper-Rank 4 Hantengu without causing herself to become a danger to her fellow Demon Slayers.
The Demon Slayer: Kimetsu no Yaiba manga began its endgame with the reveal that Nezuko is able to survive being exposed to sunlight -- the very power that's eluded Muzan Kibutsuji for 1,000 years has finally appeared in the one demon he does not have omniscient and omnipotent control over. Nezuko's power as a demon is outshined only by her complete uniqueness, as even her Blood Demon Art is harmless toward anyone who is not a demon. Her big moment is just around the corner in the anime, meaning she's about to break the internet like she breaks her fellow demons.