Keisuke Sannan holds the position of Vice-Commander within the Shinsengumi, hailing from Edo. He acts primarily as a consultant to chief Isami Kondou, presenting a scholarly demeanor marked by distinctive round glasses and dark brown shoulder-length hair. Sannan typically sports a dark green kimono shirt paired with a dark blue vest and black pants, later adopting a dark Western-style outfit in certain depictions. Initially appearing polite and amiable, Sannan harbors a cynical core, deeply resenting pity or condescension. A severe battle injury leaves his arm useless, shattering his swordsmanship and fueling isolation and bitterness. This limitation drives his secretive research into the Ochimizu (Water of Life), an elixir created by Chizuru Yukimura's father that transforms humans into Furies (Rasetsu) – beings with enhanced strength but vulnerable to bloodlust and mental decline. Believing he perfected the formula, Sannan consumes the Ochimizu, undergoing a physical transformation into a Rasetsu marked by silver hair and red eyes. As a Fury, he establishes a Rasetsu Corps within the Shinsengumi to bolster its power. However, he grows increasingly detached, conducting unethical experiments and endangering allies. He pressures other Fury converts, like Heisuke Toudou, to embrace their monstrous nature, though he reluctantly provides medicine to suppress bloodlust when challenged. His actions turn morally ambiguous, including implied involvement in civilian killings and secret meetings with antagonist Koudou Yukimura. After the Shinsengumi disbands the Rasetsu Corps, Sannan defects to the Imperial Army, aligning with Koudou to forge a "devil kingdom" for Furies. In *Warrior Spirit of the Blue Sky*, his betrayal culminates at Sendai Castle, where he aids Koudou in gathering Japan's Rasetsu as weapons. Confronted by former comrades, he unexpectedly turns against Koudou, slaughtering the assembled Rasetsu. He declares this a mercy, citing the Ochimizu's irreversible flaws: drastically shortened lifespans and uncontrollable bloodlust. In his final moments, Sannan reconciles with Hijikata, acknowledging his leadership and apologizing to Chizuru for past harm. Like all Furies, his body disintegrates into ash upon death. In the prequel *Reimeiroku*, Sannan appears as a strategic advisor during the Shinsengumi's early days. He mediates internal conflicts and emphasizes reliance on Serizawa Kamo's political connections for survival, demonstrating his pragmatic yet reserved nature long before his descent into obsession. His arc traces a path from intellectual idealism through fractured loyalty toward a final redemption, driven by a desperate desire to contribute meaningfully to the Shinsengumi's cause after his injury, even at the cost of his humanity.