The Exo Stranger is an Exo that roams the wild without a Ghost, but hailed from a future that the Darkness won. Her purpose and intentions were unknown, but she appears to be interested in the player's activities and keeps tabs on them when they venture outside of the City. 
The Stranger appears to be working for an unknown faction who know more about the war against the Darkness than Guardians do. She possesses much technology that even Ghosts don't fully understand; she is capable of appearing and disappearing out of reality, and her weapon has parts that "shouldn't yet exist." Those who have seen her are divided on who she is: some claim she is the remains of a forgotten Exo squadron fighting a long ago war while others believe her intervention has saved their lives. A few believe she is a hallucination caused by exposure to Vex technologies.
At an unknown time she encountered Rasputin, who recognized her but did not understand her purpose; and thus asks her how he could imitate her. This meeting may have occurred during one her numerous reappearances to stave off an unknown future, as in another point in time—or times—she observes the Traveler and the Last City falling to numerous unknown enemies, and among the dead sees herself appearing occasionally.
She became interested in the Guardian after going to the Moon numerous times, expecting the Vex and the Black Heart but instead encountering the Hive, an enemy who has much in common "with the machines than [ordinary] enemies alone." She concludes that the Moon was their "black heart", and that it led to the same end she wished to stop. After witnessing an unknown Fireteam of Guardians stopping the ritual in the Chamber of Night, calling it an "extraction", she went back in time to the rediscovery of the Hive, finding another Guardian, whose Ghost mentioned Venus. Believing this particular Guardian to be "the one", she went backwards in time again, following the Ghost to the Cosmodrome, intent on watching for her own purposes. This purpose is apparently that they have succeeded where others have failed, which may be stopping the Black Garden and the Heart. This is later borne out as being true, and she presents them with her personal weapon to aid them in the coming fight before vanishing.
Six years after her last appearance, the Stranger reemerged on Europa shortly after a Pyramid had arrived there, along with Eramis and her House of Darkness. She allied with The Drifter and Eris Morn after the two were also drawn to the moon.
The Stranger is first seen watching as the player escapes from the Cosmodrome. On the Moon she is spotted again by the player, looking down at them from the rocks that line the edge of the gorge where they are attempting to gain entrance to the Temple of Crota. When the player is about to enter the Chamber of Night to save the Traveler from a Hive ritual, she is able to send a transmission to them telling them they need to see her on Venus. There, she delivers a mysterious warning to the player. She is the one who warns them about the imminent Vex threat, and that the only way to heal the Traveler is to destroy the heart in the Black Garden.
The Stranger appears in the following missions during cutscenes:
In the mission A Stranger's Call, she was talking to an unknown person or group of persons, whose identity has yet to be revealed. Additionally, according to the flavor text of the last quest step in the "Not Forged From the Light" quest, Praedyth or his Ghost may have seen her at some point. This is implied to have been a recurring occurrence. While she is completely absent in the Taken King campaign, she plays a pivotal role in investigating the odd fate of Praedyth and the effects Oryx brought to the Vex. Eventually her rifle, not worn but enhanced by time itself, surfaces in the hands of the Future War Cult researchers as No Time to Explain, who give it to the Guardian. With contradictory clues on the rifle's origin, the truth behind it may never be known.
List of appearances