Copycat (Japanese:モノマネむすめ Mimic Girl) is a girl who lives in Saffron City. Her primary purpose is for a small sidequest, each with rewards for part of the storyline (Generation II and their remakes) or for the ability to learn Mimic (Through TM in Generation I, and Move Tutor in their remakes).
In games
Red, Blue , and Yellow
Copycat lives with her Mother and Father, who both refer to her by her nickname. She doesn't seem like anything other than a normal NPC until you buy a Poké Doll. When spoken to, she mimics Red, even so far as for the game to state Red is talking instead of herself. After purchasing a PokéDoll, she will take it in exchange for TM31, Mimic. Afterwords, she will speak to herself, under Red and Copycat, as if an actual conversation were taking place. Her room is littered with Poké Doll's, along with a television and a SNES. It is playing a Mario game, apparently.
Gold, Silver , and Crystal
In between Generation I and II, the Magnet Train was built over Copycat's house. In exchange for moving, she was given a Magnet Train Pass. Instead of just mimicking Gold, she also dresses up as well. Copycat had also lost her Poké Doll given to her three years prior in Vermilion City. The Doll was found by a member of the Pokémon Fan Club, who had wanted a Clefairy. He decides to give it back, hoping to befriend a real one some day. When it is returned to Copycat, she gives Gold the Magnet Train Pass.
Pokémon FireRed and LeafGreen
The basic sidequest is the same as in Red, Blue, and Yellow. She, however, now owns a Nintendo Gamecube instead of a SNES.
Pokemon HeartGold and SoulSilver
The concept is the same as in Gold, Silver, and Crystal.
In Anime
It is widely accepted that Duplica is based on Copycat, since both like to dress up and mimic people.