Mewtwo

Mewtwo is a psychic-type Legendary Pokémon who is a genetically mutated clone of Mew, created by scientists after the gene splicing experiments using Mew's DNA. It is a playable character in Super Smash Flash.

Character description
Mewtwo is Pokémon #150 in the Generation I Pokédex, the final Pokémon in the list not counting the generally-unobtainable Mew which is #151 in the Pokédex. It is a clone of Mew that has been altered by unscrupulous scientists in order to increase its power and ruthlessness in battle. As a result, it is visually similar to Mew: a pale purple, vaguely feline humanoid with strong psychic powers. Unlike Mew, Mewtwo has the height and mass of an adult human, and thinks of nothing but fighting and defeating its opponents.

In-game, Mewtwo was designed to be the "final" Pokémon of the Generation I games, having an immense Special stat and a very high speed stat, the highest base stat total with a total of 590, and the (at the time clearly overpowered) Psychic type. As a result, Mewtwo was an overcentralizing force that unbalanced the metagame until the next generation's Special split into Special Attack and Special Defense, and altered type interactions evened the playing field.

While Mewtwo sat as "just another legendary Pokémon" for several generations, it began a resurgence in popularity in Generation V, where it was given a signature attack in Psystrike. In Generation VI, Mewtwo then gained access to new transformations called Mega Evolutions. Like Charizard, and unlike most Pokémon who can also Mega Evolve, it has two Mega Evolution forms if it is holding a Mewtwonite X or a Mewtwonite Y as its item: the larger and bulkier Mega Mewtwo X, and the small, light, and floating Mega Mewtwo Y respectively.

In the Pokémon anime and movie series, Mewtwo is a (at the time of the first movie) one-of-a-kind Pokémon that develops a hatred for humans and the concept of subservient Pokémon, attempting to prove the superiority of clones over originals. It later has a change of heart and becomes a sort of anti-hero, mostly keeping to itself and trying to find its purpose.

In Super Smash Flash
Mewtwo appears as an unlockable character in Super Smash Flash. It is unlocked by beating Adventure mode with both and. It has many attacks from the Super Smash Bros. series. Mewtwo is one of the few characters who were introduced in  to appear in this game.

In Super Smash Flash 2
Mewtwo was previously confirmed to return in Super Smash Flash 2 as part of the initial roster for the game. However, following the release of v0.7 of the demo, the developers revamped the whole roster in order to accommodate some new, unexpected characters. Currently, Mewtwo's fate as a playable character is unknown.