Here is the full list of all 20 CO-OP : Decrypted achievements.
Collected your first orb!
Collected five orbs!
Collected ten orbs!
Collected all the orbs!
Completed the game!
Used R3D's power!
Used BLU3's power!
AFK'd for 5 minutes! Back to work slacker!
Reached chapter two!
Reached chapter three!
Beat the best time in one level!
Unlocked a costume!
Completed a level without dying!
Completed the game without dying!
Completed the game in less than an hour and a half!
Noticed your friend's disappearance!
Completed the game on Purist!
Got caught on camera!
Blew up your first turret!
Completed a level with a box!