While eavesdropping, Tanjiro watches as Muichiro threatens the swordsmith he was arguing with. He tries to defend him but the indifferent Hashira knocks him out for interfering. After waking up, the boy, named Kotetsu, explains that the "man" he saw is actually an advanced mechanical training doll, built after a swordsman who lived over 300 years ago. He explains the doll was made by his ancestors, and as he lacks a family, it is all he has to pass on to the next generation. Tanjiro agrees to protect it, but Muichiro appears, having finished his training session and causing extensive damage. However, after Kotetsu discovers it is still functional, he tells Tanjiro to train with it in order to become stronger than the arrogant Muichiro. However, training proves harsh as Kotetsu denies Tanjiro food and water when he fails to connect hits on the doll. When Tanjiro eventually faints from exhaustion, he imagines himself in the afterlife and sees a peculiar stone in the Sanzu River, enabling him to detect a scent faster than the Opening Thread. He uses this to his advantage, eventually landing a powerful strike on the doll's head, which shatters and reveals a concealed sword inside.