What forms of co-op exist in Saints Row 3?
Solution 1:
There is no offline/couch co-op. Online co-op and system link co-op exists for 2 players with both having the same functionality.
The whole campaign and everything else in the game is playable in co-op mode, with drop in/drop out possibility.
You're required to use an online pass code on your account to activate co-op mode on either the PS3 or Xbox 360.