Tokyo Police Department Chief Usui Naohiro presents a stern and demanding persona, initially barring Yako Katsuragi and Neuro from crime scenes to secure full police credit. This reflects his deep loyalty to the force and a long-standing rivalry with detective Sasazuka, originating from their school days with Tsukushi. Usui perceived Sasazuka's refusal to take a major exam after his family's murder as an insult to their competition, fueling a lasting grudge. He also openly criticizes Higuchi's perceived irresponsibility.

Beneath his strict exterior, Usui possesses exceptional leadership, rallying officers with motivational speeches. His competence is acknowledged even by antagonists like Sicks, who notes his effectiveness in crises. Following the Histerrier case, his stance softens towards Yako and Neuro; he funds Yako's meals during the HAL arc and accepts Neuro's tactical advice during the Kasai pursuit.

Usui harbors a concealed fondness for cute items like stuffed animals and flowers, reacting with visible embarrassment when mentioned. He prefers sweet foods, criticizing Sasazuka's unsweetened black coffee. An omake suggests his suitability for driving a Mini car.

His leadership intensifies during the Sicks conflict. After DR's flooding, his decisive actions minimize casualties, prompting an aborted assassination attempt by Tierra. Following Sasazuka's death, Usui mourns at his funeral before mobilizing the entire police force, including Higuchi and Tsukushi, to corner Zenjiro Kasai. Though Kasai escapes, Usui confronts Superintendent-General Kumakiri over covering up Sicks, using blackmail to force public exposure of Sicks' crimes and trap them in Japan.

Usui's background includes intense academic rivalry with Sasazuka, whom he viewed as slightly superior. An exchange student period in Britain led to a friendship with Andrew Sixson, later a crime investigator. This connection is exploited when Sixson is revealed as Sicks in disguise, humiliating the police department.