In Resident Evil: Revelations for the Nintendo 3DS, there is a bonus co-op mode called Raid Mode that you can unlock.
All levels of Raid Mode aren’t unlocked at once, you’ll unlock them slowly as you progress in the story.
Raid Mode Stages 1 through 7 – Complete episodes 1 through 3
Raid Mode Stages 8 through 12 – Complete episodes 4 through 6
Raid Mode Stages 13 through 17 – Complete episodes 7 through 9
Raid Mode Stages 18 through 20 – Complete episodes 10 through 12
Raid Mode Bonus Stage 21 – Beat the game
Check out my other article for the full list of unlockables in Resident Evil: Revelations for the 3DS.