Mewtwo

Mewtwo (ミュウツー, Myūtsū) is a psychic Pokémon who is a geneticaly mutated clone of Mew, created by scientists hired by Team Rocket's leader, Giovanni. After Mewtwo became aware of what was happening, it used its psychic powers to blow up the lab, killing everyone in the process. It is a playable character in Super Smash Flash and Super Smash Flash 2.

In Super Smash Flash
Mewtwo appears as an unlockable character in Super Smash Flash. It is unlocked by beating Adventure Mode with both Pikachu and Jigglypuff. It has many attacks from the Smash Bros. series.

In Super Smash Flash 2
Mewtwo was previously confirmed to appear once again as a unlockable character, despite not returning in Brawl. Its moveset in SSF2 was to be a bit different than its Melee counterpart. Its status in the game is currently unknown.