The Phantom Thief Kid or The Magician Under the Moonlight (Kaito Kid) is a world-renowned thief who steals treasures and gems. He is sometimes known as Kaito Kid or 'International Criminal 1412'. Despite the fact that Shinichi (Conan) does not care much about investigating thefts, Kid can be considered his archnemesis, as he is the one criminal (besides some members of the Black O) he can not seem to catch. Curiously, Kaito discovers Conan's true identity (non-canon, movies only), but keeps his secret, and sometimes Conan can even count on Kid's help to solve some of his cases. However, not even Kudo Shinichi knows about Kid's true identity. Throughout the story, Kaito Kid and Conan have crossed paths but he has always managed to escape from Conan's clutches by his special skills.

Spoiler

Kaito is the son of the magician, Touichi Kuroba, and apparently knew Shinichi's mother. Touichi Kuroba also made clues for Shinichi and Ran, who was in first grade. (from Vol. 55)

[Also, Kaito Kid appears in his own manga series by Gosho Aoyama, 'Magic Kaito'.]