Kogoro Mouri is a private detective operating the Mouri Detective Agency in Beika Town. He was previously a detective in the Tokyo Metropolitan Police Department's First Investigative Division, where he served alongside Inspector Megure. He left the police force after an incident in which he shot his wife, Eri Kisaki, in the leg to save her from a hostage situation, feeling responsible for the ordeal. Following this, he opened his own agency and became the single parent of his daughter, Ran, after his wife moved out. Although he and Eri have been separated for roughly a decade, they are not legally divorced.

In his typical day-to-day life, Kogoro presents himself as a lazy, boastful, and disreputable figure. He is a heavy smoker and a frequent drinker, often prioritizing alcohol, gambling, and his infatuation with the pop idol Yoko Okino over his detective work. His deductive skills in this state are poor, usually relying on the most obvious clues or assumptions, which frequently leads him to accuse innocent people. He has an inflated ego regarding his reputation and rarely questions why he consistently falls unconscious during cases only to solve them with a different set of deductions. The police, especially Megure, often find his awake behavior to be an embarrassment.

Despite these flaws, Kogoro has a strong sense of justice and becomes a completely different person when his family is in danger. When Ran or Eri are directly involved in a case, his demeanor shifts to one of sharp professionalism, and he demonstrates genuine deductive ability, a strong grasp of interpersonal intuition, and cunning. He is also a highly capable physical fighter, possessing considerable skill in judo from his university days and being an expert marksman from his time in the police force.

Kogoro's primary role in the narrative is serving as the unwitting front for Conan Edogawa. Because Conan cannot reveal his true identity as Shinichi Kudo, he secretly tranquilizes Kogoro and uses a voice-changing device to deliver his own deductions in Kogoro's voice. This has earned Kogoro the nationwide reputation of a genius detective, known as Sleeping Kogoro, a title he eagerly accepts as his own. He remains ignorant of the truth, maintaining his agency and public standing through Conan's intervention.

His key relationships are strong pillars of his characterization. He shares a deep, protective love for his daughter, Ran, despite often embarrassing her with his antics. His relationship with his estranged wife, Eri, is tense and argumentative on the surface, yet both of them harbor lingering romantic feelings for one another, and he has made several earnest attempts to reconcile. He cares for Conan as a guardian, even as he remains oblivious to Conan's manipulations. Throughout the series, Kogoro shows subtle development. While his fame is still largely a facade, he has solved several cases entirely on his own, proving he has latent talent in deduction that surfaces only when motivated by personal stakes. In later storylines, he takes on students who seek him out for their own reasons, further embedding him as a nuanced figure in the world around him.