This was for me the hardest side-mission in the game, and I couldn’t find any guide in steam about it, so I decided to make my own, hope it can help you guys.
This is the map you’ll have after completing the mission, as the hardest part of the mission is finding the bodies. Just follow the number from 1 to 10. I think this map will be enough to complete it but I leave a simple walkthrough to help in case you have any problem.
The Identity Theft side mission, can be started as soon as you exit the subway after destroying Penguin’s last disruptor. The first victim can be found at the location marked in the map.
Victim number 2
When I did this mission I had already finished the main story of the game, but searching on the internet I found out that this second victim will appear after you finish the first main story mission inside the museum.
Victim number 3
The third victim will only appear after you finish the story in Wonder City, and rescue the inmate following the mains story. After that you’re free to go find another victim lying in another alley of Arkham City. Again just follow the map.
After finding the body and scanning the area, you’ll find a trail of bleach that the killer used to cover the blood, just follow the trail and interrogate the suspect you find. He’ll give you the location of the true killer.
After all this just go to the location the last guy gave you and you’ll find where the Identity Theft Killer is.
When you enter the house, listen to the radio and watch the course of the story. That’s it for this mission.
I consulted gamesradar to know when exactly the victims would appear in the game. Feel like to visiting their page:[link]