Ditto

Ditto, known in Japan as Metamon (メタモン, Metamon), is a normal-type Pokémon that was introduced in generation I and is not known to evolve into or from any other Pokémon. It is one of the Pokémon that can be released from a Poké Ball in Super Smash Flash 2.

Pokémon description
Ditto has a simple appearance if compared with other Pokémon. Its light purple body has a slime-like consistency and handless "arms" protruding from its body, along with a face composed of a mouth and two small eyes that look like black dots. Ditto can change its appearance by using the move Transform and copy the target's appearance and moves. But, even doing it, Ditto can't change its face or mimic the target's strength and will also be unable to remain in a transformed state if it starts laughing. Ditto is never far from civilization or people and will also transform into a rock when sleeping to avoid attack.

In Super Smash Flash 2
As stated above, Ditto can be released from a Poké Ball. Once it is released, Ditto will use Transform and copy the appearance of the one who summoned it. After that, Ditto will attack the opponent using the moves from the one that summoned it. It is the rarest of all Pokémon that can be released from Poké Balls, even more than legendary and mythical Pokémon like Zygarde or Victini.

Trivia

 * Ditto was originally going to appear in Super Smash Bros. Melee, but was cut very late into development due to problems getting it to work. It was even listed in the official players' guide. In SSF2, it performs the function that all sources say it would have originally.