Nezuko Kamado

Nezuko is Tanjirou's sister.

[Spoiler] She was turned into a Demon by Kibutsuji Muzan. [/Spoiler]

[Spoiler] When the Demon Slayers were fighting Hantengu, Nezuko managed to overcome her weakness to the sun, and partially gain her ability to speak back. [/Spoiler]

[Spoiler] Thanks to Tamayo's work for giving her medicine, Nezuko finally returns to being a human in Demon Slayer Chapter 196. [/Spoiler]