Nintendo of Europe has confirmed that the free download codes for Mewtwo will be distributed in spring 2015 for players that register both versions of Super Smash Bros for Wii U and Nintendo 3DS. Participants who qualify before 22:59 (UK time) on January 20th, 2015 can receive both the soundtrack CD and the free Mewtwo download codes but have until March 31st, 2015 to register both games in order to get just the free Mewtwo codes. Mewtwo will be made available to purchase as add-on content later in 2015 though for those that missed registering or only purchased one version of the game.

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Source: http://www.nintendo.co.uk/Campaigns/Super-Smash-Bros-Club-Nintendo-Promotion/Super-Smash-Bros-Club-Nintendo-Promotion-937174.html