Hikaru Kodama serves as a pediatric nurse at St. Juliana Hospital, earning recognition for her energetic and childlike demeanor that mirrors her strong affinity for children. She possesses dark red or brown hair usually in twintails, blue eyes, and an appearance suggesting adolescence despite being 21; her character design sometimes features leg warmers and a hat.

Dr. Ryuji Hirasaka targets Hikaru for exploitative experiments, exploiting her vulnerability. He blackmails her using her sister, Ai Kodama, a patient at the hospital suffering severe heart conditions. Ai remains unaware that she and Hikaru lack a biological relationship, a truth concealed to protect her health. Hirasaka leverages this situation to coerce Hikaru's compliance, aiming to transform her into a "sex slave."

Within the "Kranke" storyline, Hikaru and Ai endure experimental scenarios orchestrated by Hirasaka. Ai expresses a desire to become a "real woman," initially believing her interactions with Hirasaka were dreams involving her sister. Hirasaka conducts trials involving both sisters, employing a device to induce shared experiences perceived by Ai as dreams. These experiments culminate with Hirasaka positioning himself as their savior, resolving their situation through his interventions. This narrative strand concludes as Hirasaka departs for the countryside with his bride, Ren Nanase, leaving Hikaru and Ai behind.

Hikaru's arc depicts her transition from an innocent, enthusiastic nurse into a victim of psychological and sexual manipulation. Her relationship with Ai remains central, serving as the key to her exploitation. The resolution in "Kranke" suggests an end to direct victimization, though her long-term recovery or future progression remains unaddressed.