The character known as Kazuhiko Amamiya, Yosuke Kobayashi, or Shinji is the central figure of MPD Psycho, a man whose identity is fractured into multiple distinct personalities following a profound psychological trauma. The original identity is Yosuke Kobayashi, an unassuming and ordinary police detective leading a conventional life with his girlfriend, Chizuko. This stable existence is shattered when a sadistic serial killer named Shimazu Hisashi, who mutilates his victims by severing their limbs, targets Yosuke personally. After the killer sees Yosuke on television and claims to recognize something in the detective, he kidnaps and horrifically mutilates Chizuko, sending her dismembered but still-living body to Yosuke in a cooler. The shock of this event causes Yosuke's psyche to break, giving rise to a new, psychopathic personality: Shinji Nishizono.

Shinji Nishizono is characterized as arrogant, cruel, and callous. This personality emerges when Yosuke finally confronts Chizuko's killer, and Shinji coldly shoots Shimazu. Following the murder, Yosuke's original personality effectively collapses and is replaced by Kazuhiko Amamiya, a calm, serious, and highly intelligent criminologist. It is Amamiya who is arrested and imprisoned for the killing. A simple physical marker distinguishes these two primary personalities: Kazuhiko Amamiya is nearsighted and wears glasses, while Shinji Nishizono does not require them, and his eyes are often depicted differently.

Upon his release from prison, Amamiya is recruited by Machi Isono, a sharp and perceptive female criminologist who runs a private investigative agency from her apartment. Amamiya is officially employed as a clerk and janitor, but unofficially, he works with Machi as a profiler, using his unique insight to solve bizarre and violent crimes. Here, the dominant personality is Amamiya, a man who is passive, deeply concerned with atonement for the actions of Shinji Nishizono, and troubled by his own fractured sense of identity. Machi is aware of his condition and the danger he represents, but she sees his abilities as invaluable, and a complex, trusting professional relationship develops between them as they work to uncover the truth behind a series of seemingly connected cases.

As the narrative progresses, it becomes clear that Amamiya's condition is not merely psychological but is the result of a vast, technologically advanced conspiracy orchestrated by a mysterious organization known as Gakuso. The different personalities are not natural dissociations but are artificially created "program personalities" designed to serve a specific function. Amamiya's role is to act as a stabilizer for these volatile constructs. The true nature of the identities is further complicated by the existence of the original Kazuhiko Amamiya, a separate, malevolent individual with burn scars on his face who was supposedly killed in a fire at the Amamiya Clinic years ago, and who serves as the basis for the program personality. Other related entities include Nishizono Tetora, a teenager who looks like a younger Amamiya and in whom the Shinji Nishizono personality is far more dominant.

Throughout the story, Amamiya's personality fluctuates, most commonly between the thoughtful, glasses-wearing criminologist and the ruthless Shinji Nishizono, especially when he is faced with danger or extreme violence. His key ability is the profound insight into the minds of criminals that his own fractured psyche grants him, but this is a double-edged sword, as his other personalities are capable of extreme and lethal action. His development centers on his quest for self-understanding and atonement, as he investigates the truth behind Gakuso, the Amamiya Clinic, and the meaning of his own manufactured existence, ultimately leading to a confrontation with the original Kazuhiko Amamiya.