She's a demon, who can't say anything other than "Mmhh". His brother is Tanjiro Kamado, a demon hunter. While Nezuko initially starts off like any other demon, bloodthirsty and hungry for human flesh, after seeing her brother’s desire to protect her from being killed by a Demon Slayer despite her being a demon, she changes completely and becomes fiercely protective of him. Nezuko seems to have forgotten a good portion of her memories as a human, besides those related to her family.