Rui

The demon ruling Mount Natagumo, who forces other demons to play the roles of father, mother and sibling in a counterfeit family, torturing them when they disappoint him. He binds slayers with near-invisible threads and puppets their bodies, wiping out most of a squad before Tanjiro arrives. Beneath the cruelty lies a child's desperate longing for the bonds he destroyed.

Rui keeps the small, frail frame he had in life, pale and thin with large reddish eyes and dark hair that fades to a pale tone at the ends. A faint spider-web pattern marks his face, and he dresses in a simple light kimono that makes his delicate build look almost fragile. The impression is deliberately pitiable, which is how he disarms his victims. When enraged his features distort: pupils narrow, limbs lengthen and harden, and his shape shifts toward something arachnid, with webbing of his own blood spilling from his body as thread.

Rui is defined by a single obsessive craving: he wants a family, and he is willing to manufacture one through terror. He assigns demons roles as mother, father and sister, demands affection on command, and punishes the smallest failure with torture or death, all while insisting that the bonds he enforces are real. He speaks in a soft, childlike register that collapses instantly into screaming rage when contradicted. Underneath the cruelty is genuine fear of abandonment and of pain, and when his illusion is broken he reverts to helpless pleading. He feels no connection to his nominal superiors and treats other demons purely as props in his domestic fantasy.

Rui was born human with a body so weak that he could barely stand, and the illness was expected to kill him. A chance encounter with Muzan Kibutsuji and a transfusion of demon blood cured him entirely, replacing his frailty with inhuman strength and a hunger for human flesh. His parents, horrified when he began killing, resolved to end his life themselves. Rui killed his father in the struggle and then his mother, and only afterwards understood that he had destroyed the one thing he valued. Unable to accept the loss, he rose to Lower Rank Five and set about assembling a replacement family out of demons on Mount Natagumo, binding each of them to him with threads and fear.

1. Natagumo Mountain Arc (the nest)

   Rui is the unseen master of the mountain, directing the demons he calls his parents and sister while his threads turn captured slayers into puppets. Whole squads from the Corps vanish on the slopes before Tanjiro, Zenitsu and Inosuke are sent in to investigate.

2. Natagumo Mountain Arc (the false family)

   His control is exposed as the family breaks apart: the mother demon, scarred by his punishments, begs to be killed, and the sister flees in terror rather than face him. Rui's reaction to their failures reveals how little his idea of love has to do with affection.

3. Natagumo Mountain Arc (duel with Tanjiro)

   Rui confronts Tanjiro directly, using threads fine enough to be invisible and sharp enough to behead. Tanjiro's blade is severed and he is driven to the edge of death, until a desperate technique combining his breathing with a rotating strike lets him cut through Rui's hardened neck.

4. Natagumo Mountain Arc (conclusion)

   Having regenerated and dropped his pretence, Rui is finished by Giyu Tomioka, whose water technique removes his head before he can react. As he disintegrates he relives his human life and the night he killed his parents, and is met in death by the family he destroyed.

Muzan Kibutsuji

Progenitor and master

Muzan's blood cured Rui's illness and made him a demon, and Rui holds a rank among the Twelve Kizuki as a result. He shows no loyalty beyond that, caring only about his own household.

The spider demon 'mother'

Coerced family member

A demon forced into the role of his mother and tortured whenever she displeased him. Her terror of him was so complete that she welcomed death at Shinobu's hands rather than continue serving him.

Tanjiro Kamado

Opponent

Tanjiro refuses the pity Rui tries to extract and names the bond he claims as a lie born of fear. Their duel is where Tanjiro first combines two breathing styles into a single improvised technique.

Giyu Tomioka

Executioner

The Water Hashira arrives on the mountain and ends Rui in one motion, demonstrating the gulf between a Lower Moon and a Pillar after Tanjiro has already fought to his limit against him.

• Revealing the mountain's demons as a family he assembled and controls through thread and punishment.
• Cutting Tanjiro's Nichirin blade apart with threads too fine to see.
• Having his hardened neck severed by Tanjiro's improvised rotating water technique.
• Being destroyed in a single stroke by Giyu Tomioka once he abandons his pretence.
• Recalling his human life and his parents' deaths as his body dissolves.

• He holds the rank of Lower Rank Five, among the lowest tiers of the Twelve Kizuki.
• His fight prompts Tanjiro's first blending of two breathing styles in combat.
• The entire spider family of Mount Natagumo exists only because he demanded relatives.
• His threads are produced from his own blood, making them an extension of his Blood Demon Art rather than real silk.

• Tanjiro Kamado
• Nezuko Kamado
• Zenitsu Agatsuma
• Inosuke Hashibira
• Giyu Tomioka
• Sakonji Urokodaki

• Muzan Kibutsuji
• Tamayo
• Yushiro
• Shinobu Kocho
• Kanao Tsuyuri
• Sabito

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