Matsuoka Kokuryu serves as a senior field analyst within the investigative organization Kura. His tall, heavily-built frame, green eyes, short black hair featuring a distinctive white fringe and side streaks, and a small scar above his left eyebrow form his defining appearance. He typically wears a black suit, white shirt, and dark blue tie, frequently adding a beige trenchcoat and brown gloves during fieldwork.

He maintains an aloof, stern demeanor during operations, valuing seniority and demonstrating a hands-on approach with recruits, such as his initial reluctance to let rookie analyst Koharu Hondomachi drive despite her license. He frequently voices frustration over perceived insufficient safety protocols for field teams, repeatedly advocating for sidearm clearance due to daily dangers. His conduct is significantly influenced by resentment toward former colleague Akihito Narihisago; previously neutral feelings shifted to outright contempt after Narihisago's revenge killing of serial killer Denshin Katsuyama (the Challenger). Matsuoka considers brilliant detectives morally comparable to the serial killers they pursue, viewing their methods as inherently compromised.

Matsuoka’s relationship with Hondomachi evolves substantially. Initially acting as her mentor and showing protective concern, his stance changes after two critical incidents: her near-fatal encounter with serial killer Tamotsu Fukuda (the Perforator), resulting in a head injury, and her subsequent killing of Haruka Kazuta (the Gravedigger) in self-defense. Fearing her potential instability and its impact on team safety, Matsuoka recommends her reassignment as a Mizuhanome pilot—a role involving entry into serial killers’ subconscious realms ("id wells"). Following Hondomachi’s instrumental role in resolving the John Walker case, however, Matsuoka develops appreciation for her capabilities as a brilliant detective, acknowledging the role suits her.

Within Kura’s hierarchy, Matsuoka operates above field analysts like Hondomachi and collaborates with Wellside investigators such as Funetaro Momoki. His duties involve real-time coordination with the Wellside team during id well explorations and utilizing the "Wakumusubi" tool to detect "cognition particles" indicative of lethal intent. His character name follows the series’ alcohol-inspired nomenclature, with "Kokuryu" translating to "black dragon" and referencing a sake brand.