When they created Myst and Riven, Cyan's intent was for players to feel as though they were themselves there, exploring strange worlds and gleaning information about them. Keeping the game first-person rather than third-person was the essence of this immersion. Thus, the games' protagonist is an anonymous, gender-neutral entity with no given history, and players are free to imagine themselves as the protagonist.
As the Myst story evolved, however, fans became curious about the anonymous player character. Atrus' interactions with this figure were vital to the story, which itself was becoming more tightly interwoven with our world. D'ni was depicted as being under our Earth, and being accessible from real locations on Earth's surface. The events from the games were determined to be at specific points (the 18th and 19th centuries) in Earth history.
With the line between Myst canon and real life blurring, fans began to speculate about the character's "actual" identity and role in D'ni history. Until such time as the truth is learned about him or her, the player character is referred to — in historical contexts — as The Stranger.
The Stranger's origin has long been assumed to be Earth, since even before there was any indication of such from the games. Uru: Ages Beyond Myst, however, strongly implies that the assumption was correct, and that the Stranger is therefore a human.
Somehow the Stranger discovered Atrus's Myst Linking Book, which had fallen into the Star Fissure (as depicted in Myst's opening cinematic). He or she touched its Linking panel and Linked to Myst. His or her explorations led him/her to Atrus, who was trapped in K'veer by his sons Sirrus and Achenar, on 12 December 1806 (according to official D'ni timelines). A month after freeing him, the Stranger Linked to Riven and freed Atrus's wife Catherine from Gehn. Atrus then let the Stranger fall into the Star Fissure, on faith that it would return him/her home to Earth.
About ten years later (1816) the Stranger met the couple again in Tomahna, a place on Earth close to the Cleft, in what is now Eddy County, New Mexico. There he/she became involved in a grievance of a man named Saavedro. Saavedro, wronged by Atrus' sons, exacts revenge by stealing a precious linking book; the Releeshahn Age. The Stranger helped Atrus recover the Releeshahn book from Saavedro.
Another ten years later (1826), the Stranger returned to Tomahna and saved Atrus' daughter Yeesha from her brother Sirrus, who kidnapped her in order to pursue his plan for revenge. It is implied in Myst IV: Revelation that the Stranger visited Atrus and his family several times in between 1816 and 1826.
The player character of Myst V is not The Stranger. Uru: Ages Beyond Myst takes place in 2003, and Myst V takes place after Uru. If the Stranger is indeed a typical human, he or she would have died before either game took place. The Myst Online: Uru Live storyline has revealed that the player character of Myst V was in fact Dr. Watson, former leader of the DRC. In the game, Atrus refers to the player character as "my old friend," but the Myst V strategy guide explains this as being a result of Atrus' age and senility.
We do not know even the Stranger's gender; references to the Stranger as a male are probably a result of English language custom rather than of actual information*. In one losing ending of Myst, Atrus sounds as though he mutters to himself, "He didn't bring the page!" referring to the player. If so, this is probably a slip-of-the-tongue by Rand Miller rather than a deliberately placed piece of information; however, it also sounds like "you".
In the Myst hint guide there is a small paragraph about the game's backstory, with highly dubious and contradictory details about the Myst book's discovery. This description is considered non-canonical.
The game Uru: Ages Beyond Myst revealed that the Star Fissure's destination — and thus the location where the Myst Linking Book fell — is in Eddy County, New Mexico. As it has also been revealed that the Myst games take place in the early 1800s, many fans speculate that the Stranger was either a Spaniard (Mexico), or an Amerindian native of the 19th century. Nevertheless, the Myst Book might well have changed hands after its original discovery before its first use.
Richard A. Watson has stated that Atrus spoke English with his grandmother before his descent into D'ni with Gehn and his journals were also written in English as a result of Anna's family having come originally from Europe, as mentioned in Myst: The Book of Atrus. Since the Stranger is able to read Atrus' journals on Myst Island, possibly the sole other datum we have about him is that he is able to speak and read English with some fluency.
The resurrection of Myst Online: Uru Live is the most likely outlet for further information on the Stranger.
The official strategy guide for Riven, in addition to the straight walkthrough, has a sort of journal obviously kept by the Stranger as he makes his way through Riven. It gives some ideas into his identity and personality.
With the conflicts, it can be inferred that the journal portion of the strategy guide was implied for comedy and immersion from someone other than the Stranger.