Ditto
Ditto (メタモン, Metamon in original Japanese language versions) is a Pokémon removed from Melee just prior to its release. Its supposed 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.
Ditto's only other appearences in SSBM are a trophy and another appearance found in Tournament Mode, in either Winner Out or Loser Out style tournaments. When selecting your character for these events, simply hold down L+R and then press A. Your character will appear as Ditto, and will be a random character and color when each match begins.