Catwoman (Batman: Arkham City, 2011)
A sidekick? Batman's feline-themed antagonist would scratch your eyes out for calling her that. Truth be told, for a good chunk of Arkham City, the master thief is a playable character in her own storyline – at least if you bought the game new, or ponied up for the DLC. It was a controversial move, given that Catwoman's storyline was integral to the plot. Mind you, Selina Kyle would probably have enjoyed all the fuss.