James O'Brian is a major character and a reluctant ally featured in Days Gone.
His character is reintroduced as a lieutenant and a member of the National Emergency Response Organization. He is forced into working with Deacon to find out about Sarah's fate, while Deacon helps him investigating N.E.R.O. operations.
Nothing is known about O'Brian's life prior to the outbreak, other than that he is a graduate student.
O'Brian volunteered to help N.E.R.O. at the start of the outbreak. O'Brian met Deacon St. John and his wife Sarah during the early weeks of the apocalypse. He loaded survivors onto a N.E.R.O. helicopter when the two arrive at the evacuation site. Deacon tried to get Sarah onto the helicopter, as she was stabbed, but O'Brian, while sympathetic, was unable to load anyone else.
Eventually, Deacon pulled a gun on O'Brian, with Boozer arriving to back him up, to force him to comply. O'Brian gave in but said that he could only take two of them. Deacon ultimately decided to stay behind with Boozer, who has suffered a leg injury, to keep him alive, and sent Sarah alone on the helicopter.
O'Brian's chopper was diverted before he could make it to the refugee camp, as it has been overrun. At the next site, he ensured that Sarah got proper treatment, getting her high-level treatment when it's against orders. He eventually lost contact with her when she was shipped elsewhere.
Events of Days Gone
Sometime later, it is revealed that O'Brian is still alive and working for N.E.R.O., now a lieutenant. He is part of a recon team investigating Freakers out in the wild. O'Brian is found by Deacon, whom he soon recognizes, and ultimately decides to help him find out what happened to his wife. O'Brian asks for Deacon's help in return, getting him to spy on other recon teams. None of the recon teams know each other's missions, and is being kept that way for unclear reasons by their command. From there, O'Brian gives Deacon GPS coordinates to their locations, having him listen in on the other researchers' findings.
O'Brian tasks Deacon with one last surveillance mission, revealing that he has found out what became of Sarah. The two meet in person and O'Brian gives him Sarah's medical file. He reveals that while she made a full recovery, the camp she was moved to was overrun, leaving no survivors among the refugees. Deacon, bitter and distraught, tells O'Brian to leave him. O'Brian apologizes for the circumstances and prepares to go, but Deacon then thanks him for saving his wife at the start of the apocalypse and the two part ways.
Much later, after realizing that his wife had government clearance, Deacon calls O'Brian again, telling him that she should have been evacuated. O'Brian does further investigating for Deacon, and finds Sarah's ID card, confirming Deacon's suspicions that she survived. Deacon tells him to take him to his wife, but O'Brian refuses. He reveals the base Sarah was moved to was overrun by Freakers and she was rescued by the Deschutes County Militia, made up of former soldiers with military-grade weaponry. Because the militia has rocket launchers, it is a no-fly zone to NERO. O'Brian also tells Deacon that the Freakers are evolving at an exponential rate, and that things will only get worse as time goes on. They part ways once again.
After successfully joining the militia and finding Sarah, Deacon calls O'Brian to request an airlift out of the militia's fortress, as Colonel Garret, the commanding officer, is becoming more and more unhinged each day. O'Brian has Deacon locate a missing team of four in a cave and take samples from a mutant Freaker called the Reacher. Deacon finds all but one of them dead, with the survivor dying from his wounds and a breach in his suit. O'Brian becomes emotional and orders Deacon to get out. Deacon then fights the Reacher and kills "him," taking tissue samples for O'Brian, before fleeing from an approaching horde. Deacon scorns O'Brian for his carelessness, which infuriates him, and tells Deacon that he better be ready when the time is right.
Upon calling for an airlift, Deacon first has to disable the rocket launchers around the camp so that O'Brian's team can land. As Deacon escapes with Sarah, they run into several new recruits led by Colonel Garret and Captain Kouri. Skizzo, who has joined them, recognizes Deacon and attacks him just as the helicopter arrives. Due to being shot at, O'Brian and his team are forced to abandon Deacon.
In the secret ending of the game, it is revealed that he has been infected with an evolved strain of the Freaker Virus that giving superhuman agility and strength but still enable him to maintain human intelligence. He also warns that NERO is initiating a massive purge at the survivors' settlement and there is nothing Deacon and his people can do to stop them, before departing the scene.