Image: Bend Studio / PlayStationOfficial screenshot from Bend Studio / PlayStation.
Brothers in Arm is a hidden story trophy tied to Boozer's storyline. It unlocks in a required mission in the second region, so every player gets it.
Keep following Boozer's missions after the move south. The trophy pops in the mission where you go back to see how he is doing.
The trophy text is checking up on Boozer. It unlocks in I Could Use A Hand.
Image: Bend Studio / PlayStationDays Done
Finish the main story of Days Gone.
Image: Bend Studio / PlayStationHold on Tight
Reached by progressing the main story.
Image: Bend Studio / PlayStationI've Been Waiting for This
Reached by progressing the main story.
Image: Bend Studio / PlayStationIt's Getting Cold Outside
Reached by progressing the main story.