Shadow Pokémon are Pokémon created by Cipher by artificially closing the door to a Pokémon's heart and turning them into fighting machines because of their lack of compassion and feelings. A fully closed Shadow Pokémon will not show their true characteristics, and they forget their normal moves and receive Shadow Moves in return. The human eye does not easily distinguish a Shadow Pokémon, and if they are, the Pokémon will cast a dark glow, and their data box is purple as well.
Due to the strange nature of a Shadow Pokémon there are some things they are unable of doing until their hearts open up and are purified.
Sometimes, Shadow Pokémon will enter a wild state of emotion known as Reverse Mode. In Reverse Mode, the Pokémon will cast a dark red glow and the data box will turn red. While in Reverse, Shadows will inflict damage upon themselves during battle, and the effects last even when the battle is over.
When a Pokémon is in Reverse Mode, use of items will be impossible. They can recover their HP by healing at a Pokémon Center, but Reverse Mode stays in effect.
There are 83 Shadows to be acquired and purified in Pokémon XD: Gale of Darkness.