Real and Fake

After killing Father, Giyu is confronted by the very Slayer he rescued. Believing a Kizuki has fallen, Inosuke demands a duel to prove himself and climb the ranks. The indifferent Hashira tells him to go back to training and points out that the fallen demon never belonged to the Twelve Kizuki at all. Inosuke insists he had said as much, only repeating Tanjiro's claim, then abruptly finds himself bound to a tree by Giyu's overwhelming speed. The Hashira tells him that a fighter who cannot recognize his own wounds should keep clear of battle, and leaves.

Dodging the thread that broke his sword, Tanjiro takes a long cut across his face and realizes the threads outmatch even Father's hide. He silently apologizes to Sakonji and Haganezuka as Rui's threads close around him. Before they strike, Nezuko appears and absorbs the attack, suffering many wounds. Horrified, Tanjiro carries her to safety. Seeing her emerge from the box, the Daughter realizes Nezuko is a demon. When Tanjiro apologizes to his sister, Rui trembles and asks if they are siblings, then declares that Nezuko's protective act is the family bond he has always wanted. The Daughter, panicking, claims to be his sister and begs not to be cast aside.

Rui releases a burst of threads that slice the Daughter apart along with distant trees, snarling that no one in his family ever fulfilled their role. He tells her she may earn forgiveness by killing the remaining Slayers, and she gathers her head and flees. Turning to Tanjiro and Nezuko, Rui says their bond moved him, yet he must still kill them, offering to spare them only if Tanjiro hands Nezuko over. Tanjiro refuses outright, declaring she has her own will. When Rui says he will bind her through fear, Tanjiro vows to behead him first. Realizing the boy will not yield, Rui sweeps his hair aside, exposing the mark that names him as Lower Five within the demon hierarchy.

Giyu ties Inosuke to a tree to keep him clear of the fight and confirms that Father had never been among the Twelve Kizuki. The confrontation between Tanjiro, Nezuko, and Rui intensifies.

Nezuko shields Tanjiro from Rui's attack, and Rui recognizes the sibling bond as something he longs for. After Tanjiro refuses to give Nezuko up, Rui reveals his rank as Lower Five among Muzan's elite demons.

This Mount Natagumo Arc chapter was adapted in Episode 19 of the anime. It confirms Rui as a member of the Twelve Kizuki, the elite demons serving Muzan Kibutsuji.

The chapter crystallizes the arc's central theme by contrasting the genuine love between Tanjiro and Nezuko with the fear-based bonds Rui forces on his clan.