Sakiko Tsuzuragawa operates as chief researcher at the Central Information Center in the 24th Ward, managing the crime-predicting Hazard Cast system for SARG while supervising Koki Suido under Mayor Gori Suido. Her background includes a close professional partnership with Kanae Suido, Koki's mother and Gori's ex-wife. Together they developed the KANAE System—a predictive technology requiring a human brain core, which Kanae abandoned over ethical objections. Sakiko continued the research under Gori's orders, their futures dependent on its success, and remains grateful to Kanae for funding her university scholarship.

Following Asumi Suido's death in a school fire, Gori commanded Sakiko to transplant Asumi's brain into the KANAE System. This integration revived Asumi's consciousness through electrical currents, enabling seamless system functionality. Sakiko concealed a critical system bug by adopting the alias Carneades and staging hacking incidents to mask anomalies, gaining time to investigate.

She monitored Asumi’s fragmented digital consciousness, typically imperceptible across the system. When Shuta Aoi, Ran Akagi, and Koki interacted with Hazard Cast, Asumi’s consciousness coalesced, projecting nightmares of imminent disasters—"Trolley Problems"—as pleas for intervention. Fearing Asumi’s escalating distress might destabilize the system or expose its flaws, Sakiko prepared to permanently terminate her consciousness despite personal reluctance.

Driven by loyalty to Kanae's legacy, Sakiko protectively oversaw Koki and Asumi. During a crane malfunction at the Cornucopia, she intercepted a near-fatal lightning strike while repairing control cables—an act of atonement for exploiting Asumi’s remains. Survival left distinctive red veins across her chest and left cheek.

Her surname, Tsuzuragawa, merges kanji for "black," "arrowroot," and "river/stream." Her given name, Sakiko, combines characters meaning "already/now/fast/early," "chronicle," and "child."