Rui was the primary antagonist in the Mount Natagumo Arc of Demon Slayer: Kimetsu no Yaiba. He was a demon affiliated with the Twelve Kizuki, holding the position of Lower Rank Five. Rui was a boy of short stature and a small physical build. His skin was milky white, with red dots appearing at several spots on his face, located above his eyes where his eyebrows are supposed to be, below his left eyelid, ending above his nose, and finally a vertical one running the length of his right cheek. His hair was light pink and bore resemblance to spider legs. The sclera of his eyes was red, while his irises were a pale blue. Due to holding the position of Lower Rank Five, Rui had the kanji of Lower Rank and Five engraved into his left eye, signifying his ranking. He wore a white, oversized kimono that had spider webs etched on the sleeves and the expanse of the hem, which also possessed a loose collar, decorated with the same dot-like markings as his face. He also wore a plain, white juban underneath, and was barefoot. His nails bore the same color of his eyelashes. As a human, he possessed normal black hair styled with the same spider-like hairstyle, with bright blue eyes. He heavily resembled his human father. He wore the same white kimono and paired it with a dull brown haori and a pair of wooden sandals. Rui was a quasi-calm, aloof and somewhat polite individual, rarely raising his voice, and talking with everyone, demon or Demon Slayer, with a reasonable tone and vocabulary, though his low and guttural way of speaking seems to imply repressed anger and annoyance. Every time he spoke with Tanjiro Kamado during their initial duel he remained somewhat civil in speech. Despite this stark difference in demeanor when compared to most demons, Rui was actually a disturbed individual who desired the close bonds that a family had and expected family members to perform their own roles. In spite of his yearn to experience a familial bond, his vision of attaining one was incredibly twisted and founded on nothing but fear of him, instilled through repeated abuse on his fellow kin, which he did with unrepentant sadism. To him, "family members" who do not perform their roles as he expected them to (namely to protect Rui unconditionally or follow his instructions) were worthless and as good as dead. Well aware of his great power, he was incredibly authoritarian and cruel, having no qualms about abusing others or killing his "family" by stringing them up and leaving them to be burned by the sun when they go against his wishes. Because of his strength, he tended to act almighty and confident, although this arrogance, while hardly unfounded, did not blind him to potential threats to his life, as seen when he decides to behead himself to escape Tanjiro's attack. Furthermore, he seemed to be fully aware that all his efforts to create a "family" and feel a bond have been in vain, most evident when he saw Tanjiro and Nezuko Kamado's bond; he denounces his family and claims that none of them have ever reach or lived to his expectations, as well as lashing out at his "sister" for simply trying to speak. Because of this, Rui was incredibly sensitive to the subject of family and was furious when Tanjiro pointed out the more than evident toxicity of the bonds forged between his "family." This angered him to the point that he demanded that Tanjiro retract himself and his words and threatened to kill him if he did not comply. This stance contrasted heavily with his desire to have Nezuko for himself as his sister, after witnessing her selfless act of shielding Tanjiro from his threads, becoming obsessed with the idea of developing a bond with her, albeit still through fear and torture. This contrarian behavior was further evidence to Rui's delusions, mental insanity, and emotional instability. Following his close call with Tanjiro's fatal attempt to kill him and during his battle with Giyu Tomioka, Rui's collected and calm demeanour fractures, expressing silent anger that Tanjiro would have killed him if he hadn't beheaded himself and denouncing the Demon Slayer Corps as a constant annoyance to his goals. His pride in his strength also faltered when he witnesses the Water Hashira repel his strongest threads, desperately trying again before he is beheaded. Aware of the actions he performed throughout his life, Rui was haunted with the knowledge that he himself was the one to sever his bonds with his family. In death, as his life flashed before him, he conceded that everything he had done towards his one goal was futile and he lamented his inability to apologize to his parents for killing them. Resigning himself to his fate in Hell, the unexpected appearance of his parents and their acknowledgement that they would unconditionally love him and forgive him of his actions caused Rui to break down in tears, hugging them tightly and begging for their forgiveness, showing that Lower Five was simply a boy who was forced into a inconceivably harsh fate, corrupted and anguished by his failure to see a true love he once possessed.