Saki acts as a juvenile informant and sidekick to detective Soutarou at the financially struggling Ootsuji Detective Agency. She possesses maroon shoulder-length hair, brown eyes, fair skin dotted with freckles, and typically wears a cap, presenting a childlike appearance.

Facing hardship, Soutarou subcontracts a critical investigation to Saki: mysterious disappearances involving individuals and trams, with clues hidden within "the Dark" – a perilous, shifting underground tunnel network beneath the city. Saki accepts this hazardous assignment, driven partly by the agency's need for payment after a previous client vanished without settling his debt.

Saki plunges into the subterranean Dark to pursue the case but subsequently disappears. Her vanishing forces Soutarou to enter the Dark himself, where he confronts anarchic conditions, warring factions, and the formidable Demon 463 armored train.

Saki's actions directly initiate the core plot of Soutarou's descent into the Dark. Her narrative role centers on her function as an informant whose own disappearance compels the protagonist's journey.