Mewtwo (ミュウツー, Mewtwo) is a fictional creature from the popular and expansive Pokémon franchise. It is introduced as the "final Pokémon" in the very first game in the franchise. Mewtwo enjoys a good degree of fame and popularity in the Pokémon fan community for its significance as the original "Über Legendary", and it is included in Super Smash Bros. Melee as a playable character.
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 retains Mew's feline features and strong psychic powers, yet sports a gray and purple coloration and larger humanoid physique due to the alterations to its genetic source material. Aside from physical differences, Mewtwo's mentality is also noticeably different from Mew's due to the aforementioned alterations, as it said to think of nothing but fighting and defeating its opponents.
In-game, Mewtwo was designed to be the "final" Pokémon of the Generation I games; it sports an immense Special stat and a very high Speed stat, the highest base stat total with a total of 590 (which increased to 680 once Generation II reformatted stat distributions to the standard used today), and Psychic as its typing (a typing notorious for being difficult to contend with in Generation I). As a result, Mewtwo was an overcentralizing force that unbalanced the metagame until the aforementioned reformation to stat distributions split the Special stat into Special Attack and Special Defense, and altered type interactions (which were also brought about in Generation II) evened the playing field.
While Mewtwo sat as "just another legendary Pokémon" for subsequent Generations after its debut in Generation I, it saw a resurgence in popularity beginning in Generation V, where it was given a signature attack in Psystrike. In Generation VI, this resurgence resulted in Mewtwo being among the initial handful of Pokémon to gain access to the new Mega Evolution transformation. Unlike most Pokémon who can also Mega Evolve, it has two Mega Evolution forms, a trait shared with Charizard. In Mewtwo's case, it will Mega Evolve into the larger and bulkier Mega Mewtwo X while holding Mewtwonite X, which also changes its pure Psychic typing to a mix of Psychic and Fighting. Conversely, it will Mega Evolve into the smaller and lighter Mega Mewtwo Y while holding Mewtwonite Y.
In the Pokémon anime and movie series, Mewtwo is, at the time of the first movie, a one-of-a-kind Pokémon that develops a hatred for humans, the concept of subservient Pokémon and attempts to prove the superiority of clones over their original counterparts. It later has a change of heart and becomes a sort of anti-hero, mostly keeping to itself and trying to find a purpose to its existence. Mewtwo is heavily based on this anime portrayal (particularly the Japanese version via its voice actor) in both of its appearances in the Super Smash Bros. series, similarly to how Lucario is heavily based on Sir Aaron's Lucario. Despite the species' purported one-of-a-kind nature, other Mewtwo have appeared across the various Pokémon continuities ever since.
Mewtwo does not appear in the original Smash in any sort of capacity. However, according to a page on the Japanese Super Smash Bros. site, it was planned for inclusion in the roster. Unlike Bowser and King Dedede however, no development was made for Mewtwo in regards to playability. 
As a playable character
Mewtwo made its Smash-series debut as an unlockable character in Melee. It is much more tedious to unlock than most other secret characters in Melee, however; it can be unlocked after either playing 20 cumulative hours of Vs. Mode matches (5 hours for four players), or playing 700 Vs. Mode matches.
Mewtwo's playstyle is very floaty and includes a decent variety of combos, strong throws, a very good wavedash, above-average dodges, a strong projectile, and one of the best recoveries. On the other hand, it has a bad combination of large size and light weight, which results in it being an easy target to hit and combo while being especially easy to KO, giving Mewtwo some of the worst survivability in Melee despite its recovery and defensive prowess. In addition to this crippling problem it has general awkwardness in its moveset and movement, most of its moves being not very useful or outright near useless attacks, lacking solid KO options, and having slow movement speed. As a result, Mewtwo has infamously sat in the bottom tier of Melee's tier list for most of the game's lifespan, being among the game's most unpopular characters and considered the worst character at several points in the game's lifespan, having only recently climbed its way out of the bottom tier to its current position of 21st.
Mewtwo, as a playable character, has three trophies - a main trophy acquired by defeating the Classic mode with Mewtwo on any difficulty, and the two Smash trophies are by beating the Adventure and All-Star modes, respectively. Its Classic mode trophy reads:
Despite not being a clone at all like the other cut characters, Mewtwo did not return in Brawl as a playable character, making it only cut character that is not a clone. This resulted in a significant amount of fan outcry, despite Mewtwo's lack of popularity as a playable character in Melee, as the majority of players saw it as an unjustified cut.
Mewtwo appears in Brawl as a collectible trophy, obtained by completing All-Star Mode on Intense difficulty. The trophy describes Mewtwo's appearance in the Pokémon games, and makes no reference whatsoever to its appearance in Melee:
While Mewtwo does not appear as a playable character in Brawl, there are several empty files on the game disc bearing its name. These include a graphic effects file (
During development of the game, an appearance of Mewtwo's Mega Evolution was reportedly considered by Masahiro Sakurai. During the Pokémon X and Y Developer Roundtable at E3 2013, an audience member asked if developers wanted to see Mega Mewtwo in the next Smash Bros. While the developers stated that they had nothing to report about the question, Sakurai, who was in the audience at the time, reportedly said "We are thinking about it." Despite this, Mewtwo did not initially make the final cut of the game as a playable character, returning only as a collectible trophy. Mega Mewtwo X and Mega Mewtwo Y also appear as a trophy in the Wii U version.
Mewtwo was later announced in the Super Smash Bros. for Wii U 50-Fact Extravaganza to be a playable character. It is the game's first downloadable character; consumers who registered both the 3DS and Wii U versions of the game on Club Nintendo before March 31, 2015 could get it for free as of April 15th, while other players were able to purchase it starting from April 28th. Its moveset is mostly unchanged from Melee, aside from the addition of a Final Smash.