Hajime Saitō, born Yamaguchi Hajime in Edo's Musashi Province, saw his father Yamaguchi Yūsuke serve as an ashigaru for the Akashi Domain. After accidentally causing a hatamoto's death, he fled Edo in 1862 for Kyoto. There, he instructed swordsmanship at Yoshida's dōjō, blending techniques from Ittō-ryū, Mugai Ryū, Tsuda Ichi-den-ryū, and Sekiguchi-ryū. Physically imposing at roughly 5 feet 11 inches, Saitō carried himself with formal posture and meticulous grooming. He aligned with Kondō Isami's Mibu Rōshigumi faction, later the Shinsengumi. His silent, intimidating presence and unmatched swordsmanship secured his role as Captain of the Third Unit, overseeing counterintelligence missions like infiltrating Itō Kashitarō's breakaway faction—operations that precipitated the Aburanokōji Incident. He engaged in pivotal clashes including the Ikedaya and Kinmon Incidents. Throughout the Boshin War, he fought as Yamaguchi Jirō at Toba–Fushimi and Kōshū-Katsunuma. After the Shinsengumi withdrew to Aizu domain, he commanded their Aizu detachment. Leading a small force at Shirakawa, he continued battling alongside Aizu troops after splitting from Hijikata Toshizō. Presumed dead at Nyorai-dō, he survived to join Aizu's Suzakutai unit. Following Aizuwakamatsu Castle's collapse, he endured imprisonment as Ichinose Denpachi in Takada Domain. Adopting the name Fujita Gorō, Saitō later worked as a Tokyo police officer during the Meiji era, executing covert assignments for authorities. He succumbed to stomach ulcer complications aggravated by heavy drinking in 1915, receiving burial at Amidaji in Aizuwakamatsu per his final request. In narrative depictions, Saitō embodies loyalty and stoicism, pursuing missions without hesitation. His reticence drives him to internalize struggles, though interactions slowly nurture greater emotional expressiveness. As a Fury—augmented by the Water of Life elixir—he acquires rapid healing and strength while facing a shortened lifespan. Amid the Shinsengumi's downfall, he battles beside comrades like Heisuke Tōdō, consuming the elixir to complete his Fury transformation. This choice empowers him to shield allies during decisive fights, surviving while many fall. His post-war existence channels duty into law enforcement.