In Tekken Tag Tournament 2 for the PS3, Wii-U, and Xbox 360, there are a lot of playable characters on the character roster, but there aren’t very many hidden unlockable characters.
So far the only hidden character I’ve seen that can be unlocked is Combot.
To unlock Combot, you need to clear Fight Lab mode, which is the mode where you use Combot through a training program to help you learn all of the different moves and techniques in the game.