Hisashi Shimazu is a pivotal antagonist whose actions serve as the inciting incident for the entire narrative. He is introduced as a brutal and manipulative serial killer, known for dismembering his victims and arranging their body parts in a decorative fashion. His methods are not merely violent but are presented as a form of grim artistry, a compulsive ritual that defines his psychopathy. A distinctive and unsettling aspect of his character is his connection to a mysterious bar code, which appears in the lower part of the sclera, or the white, of his eye. This mark is implied to be linked to a deeper conspiracy and is a tangible sign of his transformed, inhuman nature, driving him to become a special kind of psychopath who seeks to escape the confines of an enclosed society.

Shimazu’s primary motivation stems from a desire to challenge and expose a perceived kinship with detective Yosuke Kobayashi. He deliberately kidnaps Kobayashi’s girlfriend, Chizuko, and sends her to the detective in a cooler, still alive but with her limbs severed, as a direct provocation. This act was not random; Shimazu explains that he targeted her specifically because he saw in Kobayashi the same latent murderous potential that he himself possessed. In his twisted perception, the confrontation was a test to prove that Kobayashi was inherently a killer like him, thereby validating his own existence.

Shimazu’s role in the story is that of a catalyst. His death at the hands of Kobayashi fractures the detective’s psyche, leading to the birth of the series' protagonist, the alter personality Kazuhiko Amamiya. However, the circumstances surrounding his death are complex. It is revealed that it was not Kobayashi, nor Amamiya, but a third, more dangerous personality named Shinji Nishizono who actually killed Shimazu, setting the stage for the psychological turmoil that defines the series. Even after his death, Shimazu’s presence lingers. His registration with a shadowy eye bank, tied to his barcode, connects his murder to a larger web of institutional conspiracy involving organ harvesting and the implantation of the barcode technology in various individuals, including Amamiya himself.

Hisashi Shimazu is therefore more than just a criminal to be caught; he is a mirror and a creator. By destroying Kobayashi’s life and forcing a confrontation, he unmakes the ordinary detective and forges the broken, multiple personality detective who must now navigate a world filled with similar barcode-wielding killers. His actions represent a permanent rupture in the protagonist’s life, transforming a personal tragedy into a relentless, professional obsession with the darkest corners of the human mind.