In Mortal Kombat 9 for the PS3 and Xbox 360, there are two hidden characters that can be unlocked in the game. Both characters can only be unlocked by playing through Story Mode.
Here are the unlockable characters in Mortal Kombat 9, and how far you must get into Story Mode to unlock them.
Unlock Cyber-Sub Zero – Beat chapter 13 in Story Mode
Unlock Quan-Chi – Beat chapter 16 in Story Mode