Heisuke Tōdō serves as the captain of the Shinsengumi's 8th Division, holding the distinction of being the youngest member to achieve such a rank. A native of Edo, his background is marked by complexity, as he is suggested to be the illegitimate child of a lord from the Tōdō domain, receiving a monthly stipend on the condition that he stay away from his father's family. This history fostered deep-seated insecurities about his own worth and a longing for genuine familial bonds, which he sought within the surrogate family of the Shinsengumi.

In personality, Heisuke is predominantly cheerful, energetic, and remarkably talkative, often using humor to lighten tense situations. He is part of an informal trio with fellow captains Sanosuke Harada and Shinpachi Nagakura, with whom he enjoys drinking and brawling, frequently disregarding rules and curfews. Despite his jovial and somewhat childish exterior, he possesses a strong moral compass and can become serious when his principles are challenged. He is known to be hot-headed in battle and sometimes speaks without thinking, yet this same innocence and earnestness makes him approachable and well-liked. Beneath his optimistic facade, he grapples with feelings of inadequacy, particularly regarding his combat abilities which are considered the lowest among the captains, and a deep fear of death and abandonment.

Heisuke's motivations are heavily tied to his loyalty and his search for purpose. He respects Kondō Isami as a father figure and values his friendships immensely. However, his idealism and naivety make him susceptible to manipulation, as seen when he follows his former mentor, Itō Kashitarō, out of the Shinsengumi due to political disillusionment. Itō exploited Heisuke's loyalty, using him as a tool to gain influence before establishing a breakaway faction known as the Guardians of the Imperial Tomb. Heisuke’s central struggle is reconciling his personal ideals with the harsh realities of the organization he loves.

His role in the story is defined by this crisis of conscience and his subsequent tragic fate. After leaving the Shinsengumi, he is caught in the conflict between his new and old comrades. During the Aburanokōji incident, where Itō is assassinated, Heisuke is critically wounded while protecting the protagonist, Chizuru Yukimura. On the brink of death, he is given the Water of Life, a demonic elixir that transforms him into a Rasetsu, or Fury. This transformation grants him enhanced speed, strength, and regenerative abilities, but at the cost of a drastically shortened lifespan and a constant, maddening thirst for blood. A key part of his character development is his fierce determination to cling to his humanity after becoming a Rasetsu, refusing to drink blood and relying on a special medicine to suppress his urges, a struggle that contrasts sharply with others who embraced the power.

Key relationships are central to his journey. His friendship with Harada and Nagakura is a source of constant support, and they attempt to persuade him to return to the Shinsengumi after his departure. His bond with Chizuru is particularly significant; he is one of the few who treats her as an equal, and her act of saving his life creates a profound connection. In the end, Heisuke spends his remaining strength to protect his friends, sacrificing his own life in a final act of loyalty and courage that defines his maturity. In terms of abilities, aside from his swordsmanship in the Hokushin Ittō-ryū style, his most notable trait is the Rasetsu transformation, which becomes a double-edged sword that fuels both his power and his personal torment.