Ditto is a Pokémon removed from Melee just prior to its release. Its supppsed purpose was to create an AI copy of the user's character, which would aid them in battle. However, processing problems forced its removal.
It can be accessed from Action Replay, but all it does is sit there, say "Ditto!", and dissapear. The only thing it does is deal around 7-8% Damage to whoever touches it.