Yōhei Nikaidō served as a Private First Class in the Hokkaido 7th Division alongside his older twin brother Kouhei during the Russo-Japanese War. He evacuated wounded soldiers like Lieutenant Tsurumi and Sergeant Tsukishima using sledges near Sugimoto at the 1905 Battle of Mukden. Physically identical to Kouhei with short black hair and military attire, Yōhei wore puttees while Kouhei used gaiters. Yōhei exhibited a volatile, violent temperament against Kouhei’s restraint. At a soba restaurant ambush, Yōhei demanded immediate execution as Kouhei struck Sugimoto with a rifle butt. During Sugimoto’s subsequent torture, Yōhei proposed cutting out his guts to test regeneration or severing fingers individually. This aggression provoked Sugimoto to headbutt Yōhei, dislodging a front tooth and creating a permanent distinction. Consumed by vengeance, Yōhei plotted to murder Sugimoto in his cell. He ordered Kouhei to guard outside while he staged a fake escape attempt using his bayonet. Kouhei confiscated Yōhei’s firearm, advising against gunshots. Unaware Sugimoto had stolen the bayonet, an unarmed Yōhei was overpowered and killed in the struggle. Lieutenant Tsurumi later deduced Sugimoto used Yōhei’s intestines to simulate his own death. Yōhei’s death triggered Kouhei’s obsessive revenge pursuit. Kouhei preserved Yōhei’s severed ear as a relic and later attributed his own lost body parts to Yōhei, reflecting profound psychological damage from his twin’s loss.