Kozaburo Arase, a former yakuza member, now serves as a detective in the Shinjuku Police's community safety division, specifically within the third Materials Compiling Group (Comps-3). His significant underworld background preceded an undisclosed departure from that life. Arase leveraged his criminal connections to conduct numerous illegal covert operations, kept secret from the public. These activities ultimately resulted in disciplinary measures, including reprimands from senior officer Danjo Tozawa. Arase has short black hair, brown skin, and a lean, athletic build. He typically wears a black shirt tucked into formal trousers, sometimes adding a grey jacket. A large yakuza tattoo dominates his back, featuring a tiger superimposed over floral patterns and flowing water. Its exposure, such as during a police station medical exam, causes distinct discomfort among his colleagues. Personality-wise, Arase is often likened to a "wild animal" or "carnivore" due to his natural aptitude for violence. He embraces the "bad cop" role opposite his partner Tsubaki Iwanome's "good cop" persona, though his methods start subtly. He frequently maintains a slight, polite smile during interactions, letting his presence act as a quiet shadow to Iwanome. His capacity for sudden, forceful intervention—physically subduing suspects or delivering intense warnings—underscores his unpredictable danger. A particular vitality emerges during combat or pursuits, starkly contrasting his usual relaxed demeanor. Despite his past, Arase holds criminals in low regard and self-identifies as a "scumbag" for his reliance on violence. Key chronological events include an incident prior to the series where he violently dismantled the Sons of the Styx gang, hospitalizing eight members. This act deeply traumatized gang informant Takumi Kuruya and led to the gang's dissolution. During the main storyline, Arase confronts thugs harassing him for repayment, incapacitating them. He later encounters the reincarnated Corpse God and Misaki Sakimiya, identifying Takumi's drone during surveillance. After warning the pair to sever ties with Takumi due to his criminal associations, Misaki kicks Arase in the chest to enable their escape. The resulting shoe imprint is later matched to a crime scene by coroner Saki Aikawa. Etymologically, Arase's given name Kozaburo derives from "light" (耿), "three" (三), and "son" (郎), while his surname combines "laid waste, rough" (荒) and "swift current" (瀬).