Yoshiko Sagisaka endured a massacre during her school years when a mikura slaughtered her classmates before her eyes. As the sole survivor, her accounts of supernatural forces were dismissed by authorities, leading to confinement in Ookubo East Hospital's psychiatric ward. Trauma left her nearly catatonic, withdrawn and apathetic, spending days motionless in her hospital bed while her father, Detective Sagisaka Minoru, fought to prove her story true.

The attack awakened in her the rare ability to perceive youkai, sustained by her unshakable conviction in their reality amid widespread skepticism. This unique sight marked her as a pivotal witness to humanity’s hidden war against supernatural threats. Years later, during Eko’s endgame, she sheltered underground with other survivors as the police chief—unmasked as the Ushi-oni mikura—began consuming civilians.

Flashbacks revealed Ushi-oni as the same entity from her childhood trauma. When the beast attacked, Detective Sagisaka shoved her to safety, taking the killing blow himself. His sacrifice, paired with the conflict’s resolution, kindled her path toward emotional recovery. Afterward, she left Shinjuku alongside Detective Kure Narumi, retreating to the countryside as a symbolic step from fractured despair toward tentative healing.

Her story remains confined to the main series, with no documented appearances in spin-offs or other official media.