Illusion (ability)

Illusion is an Ability introduced in Generation V.

In battle
When a Pokémon with the  Illusion  ability switches into battle, it takes the form of the Pokémon in last place of the player's party. Illusion  will not activate if the Pokémon in last place is the same species or is fainted.

Outside of battle
Illusion has no effect outside of battle.