Seta Sōjirō endured severe childhood trauma as the illegitimate son of a Kanagawa rice merchant. His father's family inflicted relentless physical and emotional abuse, seeing him only as a symbol of shame. To survive, he adopted a constant, unnerving smile that masked his pain and unexpectedly lessened the beatings. This repression forged a near-total emotional detachment that became his defining trait. At six, he discovered the wounded fugitive Shishio Makoto. Shishio spared him because of his unsettling smile. Sōjirō secretly brought Shishio food and bandages, forming a bond. Shishio gifted him a wakizashi and imprinted the philosophy: "the strong live and the weak die." When his family discovered his aid and plotted his death, Sōjirō used the wakizashi to slaughter all five in a traumatic survival instinct. Despite inner anguish, he stood smiling in the rain afterward. Shishio then took him under his wing, training him as a swordsman and surrogate son. As Shishio's protégé and top enforcer in the Juppongatana (Ten Swords), Sōjirō earned the epithet "Tenken" (Heaven's Sword) through prodigious natural talent. His combat style focused on extreme speed and agile footwork, likely honed by forced labor during his abuse. His signature technique, Shukuchi (Reduced Earth), granted imperceptible movement, leaving only visible footsteps and enabling wall-running. He often used fractional variants like "three steps below Shukuchi." His emotional blankness erased detectable fighting intent, rendering his movements unpredictable. His ultimate technique, Shuntensatsu (Blinking Heaven Kill), merged full-speed Shukuchi with battōjutsu for an instant, painless kill. He wielded high-grade swords like the Kikuichimonji Norimune. His first major action was the undetected assassination of government official Ōkubo Toshimichi, using Shukuchi to reach his carriage. He later dueled Himura Kenshin in Shingetsu Village to a stalemate that shattered both their swords. During the Kyoto Arc, Sōjirō served as Kenshin's final opponent before facing Shishio. He initially dominated their battle with his speed, dodging Kenshin's Kuzu Ryūsen and slashing his back. However, Kenshin's defiance of Shishio's "survival of the fittest" ideology and refusal to kill weaker opponents eroded Sōjirō's emotional suppression, provoking internal conflict. When Kenshin urged him to find his own philosophy instead of blindly following Shishio, Sōjirō's repressed emotions erupted as rage and near-insanity. This loss of control allowed Kenshin to predict his movements. In the climax, Sōjirō's Shuntensatsu clashed with Kenshin's Amakakeru Ryū no Hirameki. Kenshin's technique overpowered him, shattering his sword and defeating him. Though Sōjirō interpreted defeat as proof of weakness, Kenshin countered that victory alone doesn't dictate truth and urged him to seek his own path. Sōjirō then deduced the secret of Kenshin's ultimate technique and relayed it to Shishio via Yumi Komagata, a Juppongatana member he viewed as an elder sister. He reclaimed his childhood wakizashi from Yumi and departed the group before Shishio's death. Following Shishio's demise, Sōjirō renounced violence. He embarked on a ten-year journey as a wanderer to atone for past killings and discover his own philosophy. He became a fugitive deemed unlikely to be captured. Five years later, he reunited with former Juppongatana member Yūkyūzan Anji in Hokkaido, where Anji was being escorted by Sugimura Yoshie (Nagakura Shinpachi). Sōjirō joined their group, continuing his quest for personal truth and redemption. Sōjirō maintained complex relationships: he revered Shishio as both mentor and father figure, owed him for enabling survival, and remained psychologically dependent on his ideology. His bond with Yumi Komagata mirrored a sibling dynamic, as both shared marginalized backgrounds (Yumi as a former courtesan). Kenshin, initially an enemy, became the catalyst for his emotional awakening and subsequent rejection of violence.