The Super Smash Bros. Wii U Direct that aired on October 23rd, 2014 announced that Mewtwo will be available for both the Super Smash Bros. Wii U and 3DS versions of the game in the Spring of 2015. It will be available via a patch to players that have bought both versions of the game.

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Super Smash Bros. Wii U Direct


Source: https://www.youtube.com/watch?v=j0BBGzfw0pQ