To earn the "ONE OF US" trophy in Detroit: Become human, you must allow Connor to become a deviant during the chapter "CROSSROADS".
It's unconfirmed at this point whether Markus' survival is crucial to Connor's conversion, but it certainly makes things quite straightforward if you have managed to keep Markus alive up until this point. Connor must have worked out the location of Jericho in the chapter "LAST CHANCE, CONNOR"; please see the guides here for how to make sure that happens:
When Connor eventually confronts Markus on the ship, you must walk through some conversational prompts before Connor is given the chance to break free of his programming in the same way that both Markus and Kara achieved much earlier on in the story. Break through the three points in the red wall to complete the conversion.
Regardless of how the rest of this chapter plays out you should receive this trophy on completing it, once you arrive at the flowcharts.