Setsuko Koike works as a nurse in Shonan Special Disciplinary School's medical section, characterized by her kind yet timid nature and deep empathy. Her physical appearance features chest-length black hair—later trimmed to chin-length—and brown eyes, typically clad in a nurse uniform. She plays a pivotal role in facilitating the escape of inmates Rokurouta Sakuragi, Minakami Mario, Maeda Noboru, and Yokosuka Jou. After tending to Sakuragi's injuries during the escape, she shelters the group covertly, supplying food and protection while developing romantic feelings for Sakuragi.

Following Sakuragi's death, she reciprocates Mario's affections but views him as a reminder of Sakuragi. Bound by familial duty to salvage her father's failing factory, she accepts an engagement to businessman Yasurou Norimatsu. She ends her relationship with Mario, confessing her lingering devotion to Sakuragi and acknowledging Mario as a memento of him. They share a final night together before parting; Mario later attends her wedding ceremony, where they bid farewell amicably.

As Setsuko Norimatsu, she bears a son named Takurou and works at Higashida Hospital. Despite challenges—including single motherhood and threats from a foreign criminal group targeting her family—she maintains stability. Relocating independently from her family to another hospital necessitates a separate residence, underscoring her path of sacrifice for duty intertwined with enduring ties to her past.