Yoko Yazaki is a police officer and former colleague of the protagonist. She teams up with him to investigate a string of police officer suicides connected to criminal operations. This investigation brings them face-to-face with a hypnotic female assassin orchestrating the deaths. During the confrontation, Yoko falls victim to the assassin's hypnotic control.

Under this control, she is forced to attack the protagonist with a knife while partially undressed. After this event, crime boss Genji Hakuryu targets Yoko. He poisons her using a lethal substance as retaliation aimed at the protagonist. Despite attempts at treatment, the poisoning is fatal. She succumbs in a hospital.

Her death motivates the protagonist's ongoing pursuit of justice against those responsible. Her role and story arc remain consistent between the manga and OVA adaptations.