Mouri Kogorou was one of the many unfortunate detectives lost in the shuffle due to Shinichi's unparalleled success as a crime-solver, but perhaps no one has profited more from Shinichi's disappearance than Mouri. With Conan tagging along Kogorou's services are regularly called upon in the most difficult and high profile criminal investigations, and because Conan is not taken seriously because of his being a child, Kogorou gets the credit for solving the cases. Conan uses his tranquilizer to put Kogoro to sleep, and thus revealing the truth behind the case with Kogoro's voice. That's how the nickname 'Sleeping Kogoro' comes, because he poses like a sleeping man whenever he's stating his deduction.
Kogorou is quick to jump to conclusions, a trait that seems to go against being a top-notch detective. However, Kogorou is an experienced detective. He just somehow can't seem to uncover the final piece of the puzzle.
Kogoro is seriously a drunkard, and he isn't responsible at all, which made his wife Eri left him. He also likes to play with women. Whenever people praises him for being a great detective, he often laughs, Kogoro style, and acts all snobby. But he could get serious at some times...