Shadow Mewtwo is a character in the game Pokken Tournament (pokemon fighting game) in the game Shadow Mewtwo or as i like to call him Angsty Kid, can deal out many attacks (I dunno what they are called)
too unlock Mewtwo and Shadow Mewtwo in the game you must defeat all the leagues and then go through some more battles while some girl in a gray jedi robe tells you to help her save Mewtwo with the Synergy Stone.
in the final battle against Shadow Mewtwo, Jedi girl (her name is anne)

Wants you too buy her time to chage up the synergy stone (even though if he owns you the first round you lose, if you win the first round then there is another round) pretty much after you beat him in the first round you get your Synergy allowing you to destroy Mewtwo therefor being able too play as him (sorry for the long description) ps: you dont get Shadow Mewtwo until you beat the "final" league, The Croma League.

Ralph's edit: *nods head thoughtfully*