Tanne operates as a mysterious figure in the subterranean realm called "The Dark" or "Kuragari," commanding the black iron armored train Demon 463. She leads a renegade police unit locked in conflict with underground gangs. Though her origins remain undisclosed, her conductor-style uniform implies a potential background linked to subterranean railways or related systems before assuming her current role.

Her primary engagement unfolds when detective Soutarou descends to locate his missing assistant. Tanne guides him through hazardous tunnels, aiding his investigation into mass disappearances afflicting the surface city. She exhibits deep familiarity with the labyrinthine environment, factions like the Laughing Masks gang, and advanced technologies such as large-scale hypnosis devices that manipulate memories or perceptions.

Tanne holds critical knowledge about the underground’s operations and forces behind the vanishings, revealing suppressed details about perpetrators to Soutarou. The narrative does not clarify how she obtained this information. Her motivations for assisting him remain undefined, and her backstory stays largely unexplored despite hints of a deeper history within the setting. Dialogue and visual cues position her as a guardian or established authority figure entrenched in the subterranean society’s chaos.

Her design features distinctive gray hair and practical attire suited to the gritty environment. She demonstrates competence navigating and combating steampunk-inspired machinery, including trains that transform into legged vehicles. While instrumental in advancing Soutarou’s mission and the plot, her personal history, development, and relationships beyond their partnership receive minimal elaboration.