Yukiko Miyoshi is a 35-year-old medical examiner in Section Nine of the National Research Institute of Police Science. She maintains a formidable professional attitude while conducting autopsies and possesses the unusual habit of napping on the autopsy table. Her skills extend beyond medicine to include martial arts training. Her personal life features a history of romantic misfortune.

Before the series begins, she was involved with Section Nine member Katsuhiro Suzuki, who died under ambiguous circumstances during an investigation. Following Suzuki's death, Yukiko enters a relationship with Ikkou Aoki, a newer Section Nine recruit. Their relationship advances to engagement. However, after the murder of Aoki's sister and brother-in-law—a consequence tied to the dangers of his memory investigation work—Aoki ends the engagement to protect Yukiko from similar risks.

The authenticity of Aoki's feelings for Yukiko, potentially influenced by residual memories of Suzuki accessed via MRI technology, remains ambiguous. Yukiko's own perspective on the relationship is similarly unclear. Her presence and connections significantly influence the emotional arcs of central characters, particularly Section Nine Director Tsuyoshi Maki. Maki harbors complex feelings towards her, stemming from her past with Suzuki and engagement to Aoki, implying protectiveness or unresolved tension.

Within the manga source material, her role is more substantial and integral to the character development of both Maki and Aoki. The anime adaptation presents her narrative significance as potentially reduced, framing her more intermittently as a secondary character focused primarily on her romantic connection to Aoki. Her background, family history, or specific formative events prior to joining Section Nine are not detailed. Her involvement in the live-action film, television drama adaptations, or the spin-off manga "Himitsu – Season 0" is also not detailed.