Double Helix has rotated the free character in Killer Instinct again as Thunder is now playable.
When Killer Instinct first came out last November, everybody could download the game for free and play as Jago. An update came a few weeks ago replacing Jago with Sabrewulf. The next time you play the game, Thunder is now the free character you can play as.
You can always pay for the full version of Killer Instinct to play as all six of the characters. If not, you can continue playing the game for free, but only playing as one character at a time.