Keisuke Sannan serves as a Shinsengumi Vice-Commander with the rank of Colonel, later leading the Rasetsu Corps. He maintains an outwardly kind, gentle, and intelligent facade, appearing rational and reserved. This calm exterior masks a pragmatic, manipulative nature and a severe, intimidating side that surfaces when angered; he habitually provokes others subtly, especially Toshizo Hijikata. Despite these traits, he holds a core belief in pacifism, opposing unnecessary violence against civilians or power misuse. A grave injury to his left arm during a mission in Osaka renders him unable to wield a sword. This triggers profound self-loathing, bitterness, and passive-aggressive behavior. He grows acutely sensitive to perceived slights, particularly from Itou Kashitaro, whom he feels has replaced him. Frustration over his disability causes him to withdraw, bathe privately to hide the wound, and lash out, leading many Shinsengumi members to avoid him. This period highlights his identity crisis, as he equates swordsmanship with his worth as a samurai and purpose within the Shinsengumi. Desperate to regain utility, Sannan deliberately drinks the Ochimizu (Water of Life), transforming into a Rasetsu (Fury). This restores his arm but alters his existence; he loses interest in human activities like eating or drinking sake, deeming them unnecessary. Initially viewing the transformation as a "godsend," he becomes fervently dedicated to researching the Ochimizu and deploying the Rasetsu Corps to strengthen the Shinsengumi. This zeal borders on obsession, manifesting in routes like Heisuke's as a pursuit of a "Kingdom of Fury." His methods grow ruthless, including killing defenseless enemies or experimenting on Chizuru Yukimura for her demon blood. His development varies across media. In Hijikata's route, he maintains control over his Fury nature, potentially feigning madness strategically. In others, like Saito's or Iba's, he betrays the Shinsengumi or dies in combat. A significant arc in *Tsukikage no Shou* sees him travel to Europe with Chizuru seeking a cure for Rasetsu corruption. He confronts the consequences: his body deteriorates with each use of Fury powers, shortening his lifespan. During this journey, he acknowledges his earlier pride in becoming a Fury as misguided but expresses no regret, as it allowed him to protect comrades and remain with Chizuru. Their research investigates European vampire legends as potential links to the Ochimizu's origins. Chizuru Yukimura is central to his redemption. Her unwavering faith and consistent kindness—offering snacks or checking on his well-being—anchor his humanity. He reciprocates by protecting her, eliminating threats sent by Kaoru. In his dedicated route, he shares a kiss with her while she sleeps before leaving on a perilous mission. In Europe, he states his choice to drink the Ochimizu enabled him to stay by her side, underscoring their mutual commitment to finding a cure. Physically, Sannan has shoulder-length brown hair, brown eyes, and stands 173cm tall. He typically wears a kimono and glasses, carrying a katana. Post-transformation, his appearance shifts, with eyes occasionally glowing red. His role as a "mad scientist" emerges through relentless Ochimizu research, though this diminishes in later arcs as he prioritizes reversing the serum's effects over embracing its power.