Sagizaka Yoshiko is a young human survivor whose entire classroom perished in a supernatural attack. This event left her psychologically damaged, leading to institutionalization in a psychiatric hospital after authorities dismissed her claims of mikura (techno-sorcerous demons) involvement as delusional. Her father, Detective Sagisaka Minoru, dedicated himself to proving these entities existed to validate her account.

During Shinjuku's apocalyptic events, Yoshiko was relocated to an emergency shelter with other refugees. There, the chief of police transformed into the Ushi-oni (a bull demon), triggering flashbacks that confirmed him as her original attacker. Witnessing this reversion, her father sacrificed himself by pushing her from the Ushi-oni's path, resulting in his death.

This second trauma paradoxically sparked her recovery. Following her father's sacrifice and the Ushi-oni's defeat, she regained her ability to speak and began overcoming her catatonia. In the aftermath, having lost faith in the city, she departed Shinjuku for the countryside alongside Detective Kure Narumi.