Koshigaya, a molecular biologist and university professor, dedicates his career to unraveling the secrets of a demonic virus that mutates its hosts into monstrous entities. As a primary Bio Hunter, he hunts down and neutralizes viral threats, frequently partnering with Komada—a fellow scientist infected by the pathogen who struggles to control his mutations.

His methodical, analytical approach clashes with Komada’s reliance on brute physical transformations. Koshigaya dissects outbreaks through scientific rigor, investigating virus-linked murders where victims’ livers are systematically consumed by the infected. His missions intersect with allies like Bokudo, a psychic aiding viral detection, and adversaries such as Seijuro Tabe, a corrupt politician whose descent into demonic transformation fuels chaos.

Though his lectures hint at academic expertise, Koshigaya’s past remains enigmatic. His professional interactions feature ethically complex dynamics, including romantic entanglements with students, woven into his academic environment without overt scrutiny.

The narrative anchors his motivations solely to containing the virus, charting his adaptation to escalating threats as he balances research with field operations. His partnership with Komada persists despite their diverging tactics, culminating in an open-ended resolution where the virus endures as a global peril. No expanded media explores his origins or future beyond the original manga and OVA, preserving his duality as scholar and operative, with personal conflicts secondary to his relentless mission.