Muzan Kibutsuji (female version) [2].

Background story.

Muzan Kibutsuji was an outstanding student, but she was arrogant, proud, boastful, and highly condescending. The teachers and even the principal of her old academy were fed up with her, so they transferred her to Sakamaki Academy, which turned out to be an academy entirely for vampires where betting is the rule. Your grades or merits don't matter; betting is what counts. Muzan had a good winning streak until she decided to challenge Tanjiro Kamado, the vampire prince and leader of the student council, to a poker game. Muzan lost badly, and since the school has a pet system—where if someone loses a bet and doesn't pay their debt, they become a pet, without voice or vote in anything—men are called "fidos" and women "minimas." Muzan ended up losing the bet and, unable to pay Tanjiro, became a pet.

Muzan's personality.

At first she was haughty, proud, conceited, and very condescending, but after losing to Tanjiro in a poker game and becoming a pet, she became quiet, shy, and submissive, mainly because pets are mistreated by most students.