For the uncultured uninitiated, Nezuko is a character from the popular anime and manga series "Kimetsu no Yaiba" or "Demon Slayer". She is the younger sister or "imouto" of the protagonist of the show, Kamado Tanjiro. Perhaps the best person to describe Nezuko is Tanjiro himself:

Indeed, just as Tanjiro says, you would be a fool to not recognize Nezuko's greatness. Even Yushiro, who doubted Nezuko's greatness eventually came to the realization that he made a fundamental error in his logic:

Very early on in the series, Nezuko and Tanjiro's entire family was killed by a demon, leaving only the 2 of them alive. Unfortunately, Nezuko is turned into a demon as a result of the attack. This leads Tanjiro to become a demon slayer, vowing to one day find a way to turn his sister back into a human. Tragic though this may be, there IS a silver lining here - Tanjiro now has the cutest demon imouto anyone can ask for.