Against Corps Rules (隊律違反 Tairitsu Ihan?) is the 21st episode in the Kimetsu no Yaiba anime TV series.
Synopsis
Seeing Tanjiro try to protect Nezuko even as he collapses triggers memories of Rui’s own past. Once a sickly boy, he became a demon after meeting Muzan Kibutsuji who gave him a share of his blood. But after he turns into a demon, his own parents try to kill him. Ever since then, he has been searching for a family. Seeking a bond that he can never hope to acquire...
Plot
As Rui dies, he is finally able to remember his human life. He was born with a frail body, rendering him bedridden - until Kibutsuji took pity on him and turned him into a demon, giving him a strong body. His father then tried to kill him while his mother did nothing, so Rui slew them both, thinking they weren't his real parents as parents were supposed to protect their children. His mother's last words were that she was sorry she couldn't give Rui a strong body, and his father had intended to commit suicide after killing him. He realizes too late it was a genuine bond after all, which traumatized him. Kibutsuji returned and told him it was his parents' fault for not accepting him and Rui accepted that rather then the truth. He missed his real parents so much he suppressed the memory of them, but was never satisfied with his fake family. Tanjiro regains consciousness as Rui's body collapses just before reaching him and Nezuko. Overwhelmed by the scent of grief he detected from it, he reached out to Rui. Rui apologized to his parents as he faded away, his spirit able to reunite with his parents'.
Giyu steps on Rui's clothing after his body dissolves and Tanjiro pleads with him not to. He will kill demons without hesitation but believes they are more tragic than terrifying creatures. Giyu suddenly recognizes Tanjiro and Nezuko from their meeting two years ago. Shinobu runs towards them and Tanjiro shields Nezuko with his body as Giyu blocks Shinobu's blade from striking Nezuko's neck, surprising her when he takes a defensive stance in front of the siblings. Giyu tells him to take Nezuko and run while he holds off Shinobu, which Tanjiro does. Tanjiro is suddenly kicked to the ground by Tsuyuri, the girl who completed Final Selection with Tanjiro, one of Shinobu's clanmates. Tanjiro tells Nezuko to run, only to be knocked unconscious by Tsuyuri, who then tries to kill the fleeing Nezuko. Giyu has captured Shinobu, who warns him that he is acting against Corps rules.
Kasugai crows, who had been watching, interrupt everyone by saying Tanjiro and Nezuko were to be taken into custody and brought back to headquarters, so Tsuyuri, Giyu, and Shinobu all sheathe their blades. Inosuke is found, but seethes to himself as he couldn't do anything to stop a single demon by himself, and begins to ponder on the reality of his strength. Zenitsu and the other victims of the elder son, even those now spiders, have been treated by the Kakushis, a cleanup unit of the Corps, and are to be brought to the Butterfly Mansion for further treatment. The sun rises at last. Tanjiro later awakens, tied up, before the Hashira, the lead swordsmen of the Demon Slayers.
