This page contains information on co-op in Gotham Knights, when can it be unlocked, is the game cross-platform and how many players can play at once.
Gotham Knights features co-op for up to four people at once. You can play together with friends or strangers who agree for you to join their game. You can also invite other players to your game so they can help you with your quests. The levels of joining players are temporarily aligned with the host's level to ensure balanced gameplay. If you join someone else's game, all made progress and earned experience will be added to your main account. You can also repeat quests alongside other players. The co-op is not cross-platform - you can only join the people playing on the same platform as you.
To unlock co-op, you must complete the second story mission, The Langstrom Drive. If you wish to play with a friend, both of you must complete this mission first, then the tutorial and training at Belfry. Once you've done this, opening the menu unlocks online play.