The Partner is Santa-san

Episode 566 of Detective Conan (Case Closed).

Long summary
Shigeko Ishikawa is being stalked by an unknown man. While Kogoro investigates, Conan is forced to watch over her two year old son nicknamed Santa. Ishikawa is later kidnapped by the unknown man and Conan investigates Santa's home in order to search for clues. Conan finds a feather of a White-eye and learns that Santa has been giving away figurines to people in the neighborhood. Conan realizes what the culprit's motive is after learning about the endangered counterpart of the White-eye, the Black-eye. Santa takes Conan to the home of the man where a White-eye figurine was placed. Conan reads a note and realizes the culprit is at an abandoned pet store. At the store, Conan confronts the culprit and reveals that the culprit was intending to sell the endangered Black-eye to the underground economy and kidnapped Santa's mother when he perceived the White-eye figurine to be a threat. Conan clears the misunderstanding and explains Santa was gave him the figurine in return for the candy. The culprit realizes he did a horrible thing, releases Santa's mother and prepares to turn himself in. The dealer whom the culprit has organized a meeting with attempts to steal the bird but is apprehended by Conan. Later that day, it is revealed that Santa's real name is Haruka Ishikawa and that she is a girl.