In the original Super Smash Bros., Mewtwo was the final unlockable character. Doing so required some persistence: players had to either play 700 versus matches or play versus mode for a combined total of 20 hours. Having done that, they then needed to defeat the genetically engineered Pokemon in a Smash Bros. match.
He has since been absent from the series, neither showing up in Melee nor Brawl. With the new Smash Bros. around the corner, though, could the pastel Pokemon be making a comeback?
It seems unlikely, but there are some tidbits that may provide a ray of hope. For one, the new versions of Pokemon, X and Y, contain a new form for Mewtwo. This has brought the 150th Pokemon back into the public eye. Secondly, at the Pokemon Developer Roundtable at E3, Justin Berube of Nintendo World Report asked about the Pokemon’s future in the Smash Bros. franchise and, while his question was effectively dismissed, series designer Masahiro Sakurai called out from the audience to say, “We are thinking about it.”
Source: Nintendo World Report