Rui

Rui is calm, controlling, possessive, and brutally intolerant of anyone who fails to behave according to his concept of family. He assigns strict roles to the demons around him and punishes them whenever their conduct fails to resemble the bonds he wants to recreate.

He insists that stronger family members must protect weaker ones, but his own behavior contradicts that principle. Rather than earning loyalty through care, he uses fear and violence to demand it, revealing that he understands the outward structure of a family without understanding the mutual love underlying it.

His encounter with Tanjiro Kamado and Nezuko Kamado confronts him with the genuine sibling bond he has been trying to imitate. Rui immediately covets that relationship and believes it can simply be transferred to him through force.

As Lower Rank Five, Rui is substantially stronger than ordinary demons. His physical speed, strength, regeneration, and durability allow him to overwhelm lower-ranked Demon Slayers with little effort.

His Blood Demon Art creates extremely durable threads from his body. Rui can manipulate them individually, weave them into webs, slice opponents apart, restrain targets, or create defensive networks. Cutting Thread Rotation forms a rapidly rotating cage of threads around a target, while Murderous Eye Basket surrounds an opponent with threads that contract inward.

Rui can strengthen his threads by infusing them with his blood, producing the red Cutting Thread Cage. He has also distributed portions of his power among members of the Spider Family, meaning his own strength during the Mount Natagumo battle does not represent everything he could possess if those abilities were reclaimed.

Rui commands the Spider Family on Mount Natagumo, where numerous Demon Slayers disappear after being sent to investigate. His subordinates manipulate, poison, and kill members of the Corps while Rui remains near the center of the mountain.

Tanjiro eventually encounters Rui and witnesses his abusive treatment of Kumo Oni Ane. Their argument over the meaning of family becomes personal when Rui sees Nezuko protect Tanjiro and decides that he wants her to become his sister instead.

Tanjiro pushes himself beyond Water Breathing and recalls his father's Hinokami Kagura while Nezuko awakens her Blood Demon Art. Their combined attack nearly reaches Rui, but Rui avoids decapitation by cutting off his own head first. Giyu Tomioka then arrives and kills him effortlessly with Water Breathing, Eleventh Form: Dead Calm.

Nezuko Kamado fascinates Rui because she willingly risks herself to protect her brother. Rui attempts to take her as his own sister, believing he can recreate the Kamados' bond by forcing her into his family.

Tanjiro Kamado rejects Rui's definition of family and becomes his principal opponent. Tanjiro's genuine love for Nezuko forces Rui to confront the difference between affection and coerced obedience.

Kumo Oni Haha, Kumo Oni Chichi, Kumo Oni Ani, and Kumo Oni Ane are unrelated demons whom Rui forces into his artificial Spider Family. He alters their appearances with his blood and punishes them when they fail to perform their assigned roles.

Muzan Kibutsuji recognizes Rui's strength and allows him considerable freedom despite the demon's unusual habit of distributing his power among others.

Giyu Tomioka confronts Rui after Tanjiro's battle. His effortless defeat of Rui demonstrates the difference between a Hashira and a Lower Rank demon.

Rui's human parents genuinely loved him and attempted to take responsibility after he became a demon and killed someone. Rui misinterpreted their actions as rejection and killed them, only understanding as his memories return that the family bond he spent his demon life seeking had already existed between them.