Keisuke Sannan, Vice-Commander of the Shinsengumi, combines intellectual acuity with a reserved, courteous exterior, masking a capacity for biting critique and calculated manipulation, especially toward peers like Hijikata. A pacifist driven by conscience, he resists indiscriminate violence and challenges morally dubious orders, balancing ethical rigor against the demands of his role. A mission-related arm injury, which permanently ends his swordsmanship, fractures his self-worth, fostering bitter introspection, social withdrawal, and simmering resentment—exemplified by his fixation on Itou as a perceived usurper.

Desperation to reclaim purpose leads him to ingest the Ochimizu, transforming him into a Rasetsu: enhanced but tormented by predatory urges. Assuming leadership of the Rasetsu Corps, he fixates on researching the Ochimizu’s properties, particularly its synergy with Chizuru’s demon blood. Narrative paths diverge his fate—some depict a spiral into amoral obsession, others a calculated facade of madness to manipulate outcomes.

In *Tsukikage no Shou*, he journeys to Europe with Chizuru, investigating Ochimizu’s ties to vampire lore while seeking a cure for Rasetsu degradation. Despite physical and mental erosion, he voices no remorse, framing his choices as sacrifices to protect others and atone. His dynamic with Chizuru shifts across adaptations, from guiding mentor to fraught ally, with her influence repeatedly tempering his inner turmoil and anchoring his humanity.

In lighter settings like *SSL*, he appears as a mild-mannered school nurse, his intellect intact but freed from historical trauma. Though kind yet socially reserved, he bonds with Chizuru through quiet empathy, while oblique references hint at shadows echoing his main-storyline struggles. All versions interrogate identity, sacrifice, and the costs of defying irreparable loss to cling to meaning.