Saichi Sugimoto was born in Kanagawa Prefecture on March 1, his name reflecting that date ("third month, first day"). Tuberculosis claimed his family in 1899. He burned their home and left the village to prevent spreading the disease. After two years wandering, he returned to find his childhood friend and romantic interest Umeko marrying his best friend Toraji. When Toraji confronted him, Sugimoto denied any interference and departed for Tokyo. In Tokyo, he assisted Mokutarou Kikuta by impersonating Yuusaku Hanazawa during marriage interviews with Kaeko Kaneko. There, he encountered Tokushirou Tsurumi and future members of the 7th Division. After rejecting Kaneko's proposal due to class differences, he enlisted in the Imperial Japanese Army in 1903. Serving as a private first class in the 1st Division during the Russo-Japanese War, Sugimoto fought at the Battle of 203 Hill. His ferocious style—charging trenches under fire, surviving gunshot wounds, and wielding bayonets with brutal efficiency—earned him the name "Sugimoto the Immortal." Toraji sacrificed himself to save Sugimoto from a grenade blast. With Toraji dying in his arms, Sugimoto vowed to care for Umeko, whose eyesight was failing, making this promise his driving purpose. Nearly beaten to death after the war for almost killing an officer, he was discharged, forfeiting the Order of the Golden Kite and his pension. Muscular with brown eyes and spiky brown hair, Sugimoto typically wears a military cap. Extensive scarring marks him, including parallel vertical scars on his cheeks and a horizontal branch-like scar. His body bears additional battle scars, accumulating more in Hakodate (partially torn right ear, bullet scars on his lower left face, a horizontal scar on his left ear). His attire is an olive kimono with sleeves tied by a tasuki sash, military trousers, and boots taken from the assaulted officer. In cold weather, he wears a dark blue trench coat and a foreign-made scarf. Following a head injury from Hyakunosuke Ogata at Abashiri Prison, he wore a metal plate; after Nikaidou injured his leg, he used a brace until returning to Hokkaido. Sugimoto's personality blends fierce protectiveness with deep trauma. Loyalty to Toraji's memory and Umeko's well-being drives him, yet profound guilt from wartime actions fuels a belief he deserves damnation. His mantra is "I’m not gonna lose. I’m going to live, damn it." He exhibits impulsiveness, especially defending others, worsened by brain damage from Ogata’s gunshot, increasing recklessness and susceptibility to berserker rages—frenzied states halted only by extreme physiological shocks like sudden cold. Despite his hardened exterior, he forms a deep bond with Asirpa, participating earnestly in Ainu culture and prioritizing her safety above all. He shows forgiveness, often aiding former enemies, and rejects humiliation tactics due to his own experiences with ostracization. His combat skills stem from military training, specializing in judo, kendo, and bayonet tactics. His "immortality" comes from unnaturally high pain tolerance, rapid healing (attributed to fast metabolism and damp skin), and relentless tenacity, enabling recovery from injuries like bear maulings and explosions. Wilderness survival skills developed during his travels with Asirpa across Hokkaido. His equipment includes an Arisaka Type 30 rifle, Type 30 bayonet, ammunition pouches, an oil can for maintenance, a Higonokami folding knife, and a Type 26 revolver looted from Kanjirou Kasahara. Sugimoto's pursuit of Ainu gold began while panning in Hokkaido to fund Umeko's eye surgery. Hearing the legend of tattooed convicts from the drunken Takechiyo Gotou, Sugimoto later found Gotou’s tattooed corpse and allied with Asirpa, an Ainu girl seeking vengeance for her murdered father. Their partnership involved tracing tattoos from escaped prisoners, recruiting escape artist Yoshitake Shiraishi, and clashing with factions like Tsurumi’s 7th Division and Hijikata Toshizō’s nationalist group. Alliances shifted pragmatically, such as temporarily joining Hijikata against the 7th Division or collaborating with former enemies to rescue the abducted Asirpa. The treasure hunt concluded with Sugimoto securing a portion of the gold. He fulfilled his promise to Toraji by delivering the gold to Umeko, who had regained her eyesight independently, remarried, and had a son. Sugimoto gave her the gold to ensure her family’s financial security, resolving his character arc. Sugimoto draws inspiration from two historical figures: creator Satoru Noda’s great-grandfather, a cavalryman awarded the Order of the Golden Kite for Russo-Japanese War bravery, and Hiroshi Funasaka, a WWII soldier known for extraordinary survival and bayonet skill. Noda clarified the protagonist shares only the name with his ancestor.