Ubume

Ubume is the primary antagonist in the Third Light Novel of Demon Slayer: Kimetsu no Yaiba. She was a demon affiliated with the Twelve Kizuki, having formerly held the position of Lower Rank One (の).

Multiple decades ago, Ubume was a human by the name of Yae and lived a normal live along with having a husband before turning into a demon personally by Muzan Kibutsuji.

Personality
When she was a human and a mother, Ubume was a parental figure who cared immensely for her loved ones, especially her only daughter, Sae. But after she became a demon, Ubume started becoming a more wicked and twisted person, displaying a kind and compassionate disposition on the outside, but an extremely merciless and cruel one when she fights.

Abilities
Overall Abilities: As Lower Rank One, Ubume was the strongest Lower Rank at the time and the closest to matching the strength of the Upper Ranks. She was able to stand her ground against both Sanemi Shinazugawa and Masachika Kumeno (who were Kinoe-ranked at the time), two powerful Demon Slayers trained with their respective Breathing Style, Wind Breathing, even killing the latter before she died.

Blood Demon Art
Illusion Manifestation: Ubume and cast powerful illusions at will. She initially uses it to change a room in a mansion into fleshy walls of her stomach. The illusions are so powerful that even a Kinoe-ranked Sanemi failed to sense he was under its spell and continued to struggle against Ubume but to no avail. He only escaped this illusion through Masachika, who broke the demon-dispelling mirror, which greatly weakened her illusions. She used this ability one last time on Masachika by manifesting the image of her deceased daughter, Sae, to stop his blade attack, allowing Ubume to stab him in his abdomen.

Trivia

 * Ubume is not Yae from Giyu Tomioka: Gaiden as the kanji for each of their names are completely different.
 * She has stated that Muzan is the one who gave her this name.