Imposterin Pokémon Polished Crystal

Imposter is an ability that allows the user to automatically transform into the opposing Pokémon upon entering battle. This includes copying the opponent's stats (except HP), moves, Ability, and type(s). It is only found on Ditto in Polished Crystal. While Ditto does know Transform as well, Imposter will activate before Ditto can use Transform, making it a very quick way to start the battle.