Mewtwo (ミュウツー, Mewtwo) is a fictional creature from the popular and expansive Pokémon series. Initially introduced as the supposed final Pokémon in the series' first installments, Mewtwo has since enjoyed a good degree of fame and popularity among the Pokémon fanbase, thanks to its significance as the original "uber-tier" Legendary Pokémon and for its prominence in the franchise's first movie.
Mewtwo made its Super Smash Bros. series debut in Super Smash Bros. Melee, though it was one of the five playable characters from that game to not return as such for Super Smash Bros. Brawl. Although initially absent during the launch of Super Smash Bros. 4, Mewtwo made its return to the series in that game beginning in April 2015, which also saw it become the first playable character in the series' history to be made available as downloadable content.
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 make it solely oriented for battling. As a result, it retains Mew's feline features, but instead sports a gray and purple coloration, a large, humanoid physique and even stronger psychic abilities, due to the alterations to its genetic source material.
Mewtwo's mentality also vastly differs from Mew's: the last journal entry written by a researcher who oversaw Mewtwo's origin mentions that its viciousness lead to it violently escaping captivity, while Pokédex entries in Generations I and II establish that it thinks only of defeating its opponents and supplements this intention with a propensity for focusing its powers and resting in undiscovered areas.
In-game, Mewtwo was designed to be the final Pokémon of the Generation I games and has traits that highlight its status as such: it is a Psychic-type (a typing notorious for being difficult to contend with in Generation I) that sports an immense Special stat, a very high Speed stat, and 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). As a result, Mewtwo was an overly centralizing 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) collectively brought balance to the metagame.
While Mewtwo's prominence waned in the wake of numerous other Legendary Pokémon appearing in subsequent Generations, it nonetheless acquired a signature move called Psystrike beginning in Generation V. In Generation VI, Mewtwo's prominence underwent a resurgence, due to it being among the initial handful of Pokémon to gain access to the new Mega Evolution transformation. Unlike most Pokémon who can Mega Evolve, it has two Mega Evolutions, 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 it into a Psychic/Fighting-type. Conversely, it will Mega Evolve into the smaller and lighter Mega Mewtwo Y while holding Mewtwonite Y.
In the Pokémon anime and movies, 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 antihero, mostly keeping to itself and trying to find a purpose to its existence. Despite the species' purported one-of-a-kind nature, other Mewtwo have appeared across the various Pokémon continuities ever since, including one in the sixteenth movie that is capable of undergoing Mega Evolution.
Despite being genderless like Mew, Mewtwo in the anime assume gender roles, much like other Legendary Pokémon that can speak human language. In this case, the voice for Mewtwo in the initial adaptations is often male (which is reflected in the Super Smash Bros. series), with the exception being the one in the sixteenth movie having a feminine voice.
Mewtwo does not appear in SSB in any capacity. However, according to a page on the game's Japanese site, it was planned for inclusion in the roster. Unlike Bowser and King Dedede, however, no development was made for Mewtwo in regard to being a playable character. 
As a playable character
Main article: Mewtwo (SSBM)
Mewtwo made its Super Smash Bros. 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 Versus Mode matches (5 hours for four players), or playing 700 Versus Mode matches.
Mewtwo is very floaty and includes a decent variety of combos, strong throws, a very good wavedash, above average dodges, a strong projectile in Shadow Ball, and one of the best recoveries in Teleport. 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, with most of its moves being not very useful or outright near useless attacks. To compound this, it lacks solid KOing options, and has 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. However, the efforts of Taj have resulted in Mewtwo being reassessed as a low-tier character, which is reflected with its ranking of 21st and its placement in the F tier.
Main article: List of SSBM trophies (Pokémon series)
Mewtwo, as a playable character, has three trophies - a main trophy acquired by clearing Classic mode with Mewtwo on any difficulty, and the two Smash trophies are by clearing the Adventure and All-Star modes, respectively. Its Classic mode trophy reads:
Mewtwo did not return in Brawl as a playable character, despite not being a clone like the other cut characters. This resulted in a significant amount of fan outcry in spite of Mewtwo's lack of popularity among the competitive community, as the majority of players saw it as an unjustified cut.
Main article: List of SSBB trophies (Pokémon series)
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:
Main article: Beta elements (SSBB)#Scrapped characters
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 (
Main article: Mewtwo (SSB4)
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 Super Smash Bros. game. 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 together as a singular trophy in Super Smash Bros. for Wii U.
Mewtwo was later announced in the Super Smash Bros. for Wii U 50-Fact Extravaganza to be a playable character, while its return to the series was notable for being the first time in Super Smash Bros. history that a character has been made available as downloadable content. Consumers who registered both Super Smash Bros. for Nintendo 3DS and Super Smash Bros. for Wii U on Club Nintendo before March 31, 2015 could download it for free as of April 15th, while others were able to purchase it beginning on April 28th. Its moveset is mostly unchanged from Melee, aside from the addition of a Final Smash.
Mewtwo is ranked 9th out of 58 on the SSB4 tier list, placing it in the A tier. Upon its return to the series, Mewtwo had been poorly regarded by the competitive scene, as its already light weight was lowered further and its already tall hurtbox was slightly increased in height. However, Mewtwo did see improvements to Confusion and Disable, two of its most arguably least used moves: the former now functions as a traditional reflector, while the latter's stun effect has a longer duration. Despite the aforementioned poor competitive view, Mewtwo saw a turning point thanks to game updates. Update 1.1.3 increased Mewtwo's grounded and aerial speeds, as well as improved its already potent forward smash and aerial attacks. Although update 1.1.5 provided very few changes, they were effective regardless: Mewtwo's weight was marginally increased, its already fast dashing speed was increased to the point of becoming the seventh fastest in the game, and both its up smash and neutral aerial were made safer thanks to adjustments to their ending lag and hitbox sizes, respectively.
As a result of these very substantial buffs, opinions on Mewtwo have been much more positive: it has been reassessed as a top-tier character, has managed to earn some noteworthy representation, and it consistently achieves high placings at all levels of play.