The Stranger is an unknown human from North America who lived during the 1800s CE. Over the course of nearly 20 years, they were instrumental in providing assistance to Atrus and his family during a number of crises.
In 1806 CE (9462 DE), the Stranger came across a Linking Book to Myst. Atrus dropped it into the Star Fissure while executing his plan to trap his father, Gehn, on Riven, and it fell into the New Mexico desert. They arrived on Myst to find it abandoned. Several months earlier, Sirrus and Achenar had trapped Atrus—their father—in D'ni and linked into the prison Ages of Haven and Spire, unaware that they would be unable to return. After exploring the Ages that were still undamaged, the Stranger uncovered enough clues to rescue Atrus from his prison. The events of this process were fictionalized in Cyan's 1993 game Myst.
Atrus called upon the Stranger several months later, enlisting them in his efforts to rescue his wife, Catherine, from Riven. She had been tricked by her sons into linking to D'ni as part of their plot, and proceeded to Riven thinking that she would find Atrus there waiting for her. Atrus sent the Stranger to Riven to retrieve her, as well as permanently imprison Gehn in a new prison Age from which no escape would be possible. The Stranger was successful in their mission, though their means of "signaling" Atrus that their task was complete—re-opening the Star Fissure—ultimately resulted in RIven's final and permanent collapse. The Stranger fell into the Fissure after Atrus and Catherine were reunited, and returned safely to Earth. These events were fictionalized in Cyan's 1997 game, Riven.
In 1816 CE (9472 DE), the Stranger visited Atrus and Catherine at Tomahna, the new home that they constructed in New Mexico. Atrus wanted to take the Stranger to Releeshahn, the new Age he had written for the survivors of the fall of D'ni. Unfortunately, before they could link there, the Book was stolen from Atrus's study by Saavedro—a man from Atrus's past who had been exiled from his home by Sirrus and Achenar 20 years earlier. During the theft, Saavedro set part of the study aflame. The Stranger was the only one able to reach the Linking Book Saavedro used to escape before it was burned, and pursued him. They recovered the Releeshahn Book and reunited Saavedro with his people, whom he had believed were all dead. Presto Studios released a fictionalized version of these events in 2001's Myst III: Exile.
Eight years later, in 1842 CE (9480 DE), Atrus once again called on the Stranger to help him with a family dilemma. He and Catherine were considering releasing Sirrus and Achenar from their prisons, but Atrus needed an outsider's opinion before he was willing to make a final decision. His plan was to observe the brothers through an enhanced version of his crystal viewer device, but it malfunctioned catastrophically just after he and the Stranger calibrated its sound amplifier. While Atrus was away in Rime retrieving parts to effect repairs, Sirrus broke out of his prison on Spire and kidnapped his little sister, Yeesha. He took her to Serenia, intending to swap his consciousness into her body so that he could trick his father into teaching him regestoy. The Stranger was knocked unconscious by Sirrus's incursion into Tomahna, but awoke several hours later and endeavored to rescue the young girl. Achenar died helping the Stranger pursue Sirrus into Dream, where they wrested him free of Yeesha's consciousness. Untethered, Sirrus's mind was "crushed" by Dream, effectively killing him. At the end of the ordeal, the Stranger took Yeesha back to her parents. UbiSoft's "Team Revelation" released Myst IV: Revelation, which chronicled these events, in 2004.
After rescuing Yeesha, the Stranger disappears from the historical record. While Cyan would go on to release Myst V: End of Ages in 2005, that game did not feature the Stranger as its silent protagonist. Instead, it was a fictionalization of Dr. Richard A. Watson's actions following the DRC's departure from D'ni in 2004. Atrus does refer to the player as "my friend"—the same moniker he gave to the Stranger in earlier games—but Cyan has indicated that this was some mixture of Atrus losing his eyesight in his old age, and artistic license to allow the game to fit better into the series's larger narrative. Yeesha also refers to the Stranger in her journals, remarking on the numerous times they were there to help her family in a time of great need.