In Resident Evil 6 for the PC, PS3, and Xbox 360, the lovely Ada Wong can be unlocked as a playable character in Mercenaries Mode.
To unlock Ada Wong in Mercenaries Mode, you’ll need to complete her campaign story first, which also has to be unlocked.
To unlock Ada Wong’s campaign story, you need to beat the game with the other three characters.
Check out my other article for a complete list of cheats, unlockables, tips, and tricks for Resident Evil 6.