Born in Kanagawa Prefecture in September of the first year of Bunkyū (1861), Sōjirō is an illegitimate child who was badly mistreated by his father's family to the point of being beaten for faltering in his tasks. To build a tolerance to the beatings, Sōjirō put on a smiling face as his tormenters decide to let him be. One night, at age eight, Sōjirō witnessed the recently scarred Shishio Makoto killing two police officers and was spared due to the criminal seeing the boy's smile. While secretly feeding Shishio and providing him with bandages, Sōjirō takes the man's ideals to heart while receiving a wakizashi. When his family learned that he is harboring Shishio and decide to murder the child and pin the blame on the fugitive, Sōjirō uses the wakizashi Shishio gave him to slaughter his family. Soon after, once Shishio's wounds are healed, Sōjirō travels with him as his right hand and member of the Jupongatana.