Friendship Trophy in flOw
Complete any campaign with 4 players.
How to unlock Friendship
You can do this with one controller! Using any creature, swim to the deepest part of the sea (with the egg in the middle).
On PS3: Hold down the button and re-assign the controller to another port (port 2), then hit select and you will see the new player appear. Repeat for port 3 and port 4.
On PS4: Hold down the button and select Adjust Sound and Devices > Turn Off Device > DUALSHOCK 4. Hold down again and select a different profile (you will need four PSN profiles on your PS4 total). In game, choose "Options" to spawn.
When all four 'players' are present, eat the egg (with any of the players) to finish the stage and unlock the trophy. Good teamwork!