Fusatarō Ōsawa, known as Boutarou the Pirate, hails from Hokkaido near Asahikawa. Born into a forestry family, he lost all relatives to childhood smallpox, forging a deep dread of being forgotten. This fear fuels his lifelong ambition to become a king and found a vast, enduring family to remember him. Tall and muscular, he possesses distinctive features: knee-length dark hair, angular eyebrows, sideburns, webbed hands, and 36 cm feet. The characteristic Abashiri prisoner tattoo marks his chest and back. He typically wears practical attire—a white shirt, light grey trousers, and suspenders—sometimes adding a fur-trimmed coat. Preferring barefoot operations due to his reliance on swimming for crime, guards dubbed him "The Pirate of The Prison." His criminal path began young while ferrying lumber. He drowned victims in rivers to steal their belongings, resulting in 55 recorded robberies and murders, though likely more occurred. These acts led to Abashiri Prison, where fellow convict Keiji Ueji subjected him to years of psychological manipulation. Ueji falsely claimed sightings of Ōsawa’s aunt, a lie later exposed by Tatsuma Ushiyama. After escaping prison, Ōsawa tried to kill and skin yakuza boss Kiichirou Wakayama. Wakayama spared him by convincing him the tattoo code was unsolvable, shifting Ōsawa’s sole focus to finding the physical Ainu gold. Ōsawa formed a temporary alliance with Sugimoto’s group after a failed steamboat robbery attempt. He revealed his dream of using the gold to build a Southeast Asian kingdom and raise a large family. He briefly connected with Sugimoto over shared loss of family to disease. Yet his inherent distrust and opportunism drove him to betray the group upon recognizing an associate’s belongings. This betrayal ended in an underwater struggle where Sugimoto severed Ōsawa’s hair to free him from a ship’s wheel. Sugimoto proposed realliance, acknowledging Ōsawa’s unique ability to locate the gold without the skins. During the journey, Ōsawa proved perceptive about others’ motivations. He deduced Asirpa was Wilk (Noppera-bo)’s daughter and questioned her dedication to her goals. He discovered a marked coin in Lake Shikotsu; Asirpa identified it as an Ainu symbol for "mutually" or "to cross over," hinting at collective efforts for Ainu autonomy. His interactions frequently probed others’ dreams, mirroring his obsession with legacy. In his final act, Ōsawa shielded Shiraishi from gunfire during a clash involving the 7th Division and Hijikata’s faction. Dying, he entrusted Shiraishi with the tattooed skins, revealed the Ainu gold’s location, and implored Shiraishi to ensure future generations remember his name and epithet "Boutarou the Pirate." This sacrifice transformed his self-serving criminality into a quest for posthumous remembrance. His influence endured as Shiraishi vowed to honor his memory.