Rui

"Friends? Stop being facecious. We're not in that kind of inferior relations. In truth, we are family."

- Rui as he tortured his so-called "older sister"

Rui is a supporting antagonist in Demon Slayer: Kimetsu no Yaiba and the main antagonist in its Natagumo Mountain Arc. He was a member of Twelve Demon Moons and the Lower Moon Five.

Originally a sickly child who was transformed into a demon by Muzan Kibutsuji, Rui killed his parents when the latter decided to kill him from harming others. Driven by guilt over his patricide and matricide, Rui became a disturbing person and was later adopted by Muzan and became the leader of the fabricated Spider Family, made of Spider Demons that pretended to be a family.

He was voiced by Kōki Uchiyama, who also voiced Shirou Kotomine.

Personality
Rui is a distrubing and childish individual who desired the close bonds that a family has and expected family members to perform their own roles. To him, "family members" who do not perform their roles as he expects them to (namely to protect Rui unconditionally or follow his instructions) are worthless and as good as dead.

Despite the fact he yearns for a "family", Rui's vision of one is incredibly twisted. All he wanted was to make his fellow kin to fear him, with the fear instilled through repeated abuse on them, which he does with unrepentant sadism. He also shows no tolerance towards failure and seeks to terrify and punish whoever that failed him.

However, Rui still showed an sensitive side on his ideas of perfect family. He was clearly offended when Tanjiro called out his actions, saying Rui's family was made of naught but a fake bond and pure terror and toxicity. This stance contrasts heavily with his desire to have Nezuko for himself as his younger sister, particularly after witnessing her selfless act of shielding Tanjiro from his threads. Much to Tanjiro's shock and disgust, Rui soon became so obsessed with the idea of developing a bond with her, albeit still through fear and torture.

This contrarian behavior is a further evidence to Rui's morbid delusions, mental insanity and emotional instability. Deep down, Rui was a child fear of lonliness and desired to be loved, but his standard towards love was nothing but fear and respect with he himself as the center of his so-called "family".

Trivia

 * Rui was ranked in 16th place as of the first character popularity poll with 191 votes.
 * Interestingly, this wasn't the first time with Kōki Uchiyama (the voice actor of Rui) and Natsuki Hanae (the voice actor of Tanjiro) voicing two enemy characters. They already voiced Shirou Kotomine and Sieg, respectively, in Fate/Apocyrpha (2017).