Ditto

Ditto, known in Japan as Metamon, is a normal-type Pokémon that was introduced in generation I and is not known to evolve into or from any other Pokémon. In Super Smash Flash 2, Ditto appears as one of the Pokémon that can be released from a Poké Ball or Master Ball to aid the player.

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 it will 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 or Master Ball in Super Smash Flash 2, and it is the rarest of all Pokémon that can be released. 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.

Trivia

 * Ditto is currently the only Pokémon that can be summoned from a Master Ball in SSF2 that is not a Legendary Pokémon or a Mythical Pokémon.
 * Ditto was originally going to appear in Super Smash Bros. Melee, but it 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 claim it would have performed originally.
 * Following its inclusion in SSF2, Ditto would later appear in Super Smash Bros. Ultimate, where it functions very similarly, albeit it takes on a purple gelatinous texture rather than the full appearance of the copied character.