Hikaru Kodama is a 21-year-old pediatric nurse at St. Juliana Hospital. Energetic and naïve, she displays mental immaturity and lacks conventional womanly composure. She channels intense devotion and tolerance into caring for her young patients, earning deep trust despite her youthful demeanor. Visually, she possesses dark red hair often in pigtails, blue eyes, and wears baggy stockings.

Her background centers on her adoptive sister, Ai, hospitalized at St. Juliana for heart conditions. The sisters' non-biological relationship remains concealed to protect Ai's fragile health. This secrecy becomes Hikaru's vulnerability. Ryuji Hirasaka exploits Ai's condition and the hidden adoption truth, threatening disclosure to endanger Ai and coercing Hikaru into compliance as his victim.

Throughout the series, Hikaru endures extensive psychological and sexual manipulation as Hirasaka's final target. Inhumane experiments aim to shatter her spirit and forge submissiveness. This abuse transforms her naturally feisty, confrontational personality into one marked by forced docility and compliance.

In the spin-off *Kranke*, Hikaru's degradation intensifies under further deviant experimentation. Hirasaka engineers scenarios involving both sisters, positioning himself as a "savior" by orchestrating intimate encounters between them while manipulating Ai into perceiving these acts as dreams. Consequently, Hikaru sheds her submissive demeanor, adopting a hypersexualized, unrestrained identity characterized by sexually aggressive behavior.

Her narrative concludes in *Kranke* with Hirasaka departing to pursue experiments elsewhere, leaving Hikaru and Ai permanently ensnared in the unresolved aftermath of his manipulations and their shared trauma.