Ryuji Hirasaka is a middle-aged Japanese gynecologist defined by his complete absence of ethics and fixation on psychological and sexual domination. Unemployed and living independently when the series begins, he unexpectedly receives a job offer from St. Juliana Hospital. Arriving there, he recognizes the hospital director, Narumi Jinguji, as a woman he brutally raped in their past. Despite this history, she tasks him with leading a new department designed to fulfill patients' sexual interests by converting nurses into compliant sex workers. Driven by sadistic excitement and the chance to indulge his deviant urges, he accepts the role.

His approach centers on identifying and exploiting nursing staff vulnerabilities through psychological manipulation and cold reading. He systematically targets multiple nurses: Ren Nanase, whose hidden involvement in boxing match-fixing becomes blackmail material; Remi Shinjyo, whose concealed medical malpractice provides leverage; Ako Fujisawa, falsely accused of drug theft; and Hikaru Kodama, whose adoptive sister Ai suffers from severe health issues. He subjects each to progressively extreme and degrading experiments, including paraphilic acts and psychological conditioning, aiming to break their will and induce Stockholm syndrome.

In the original storyline, these actions culminate in violent conflict. Narumi attempts to kill him for his abuses but fails, then commits suicide. Ren Nanase, driven to desperation, fatally stabs him. During her subsequent arrest, Ren admits to developing complex emotional attachments to him.

The "Kranke" narrative diverges significantly, particularly in his interactions with Hikaru and Ai. He creates an experimental drug to amplify female pheromone secretion, using it to engineer a sexual connection between the sisters—ostensibly fulfilling Ai's wish to "become a real woman" while advancing his research on psychological conditioning. This timeline concludes with him surviving the hospital conflict and marrying Ren Nanase. They relocate to a rural area, where he continues experimental practices solely on Ren under medical research pretenses. Narumi Jinguji attends their wedding, signaling a transformed dynamic from the original timeline.

Across both narratives, he consistently displays intellectual brilliance weaponized for cruelty, prioritizes psychological control, and views others as experimental subjects. While the original story ends in his violent death, "Kranke" portrays a domestic stability that perpetuates his exploitation within marriage.