Next to nothing is known about this person since it seems from the narrative that Cyan did not intend to develop the character. The Myst games have no dialogue, only monologues from non-player characters, and they never asked the Stranger about their origin.
We don't 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.
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 Spanish Mexican or an Amerindian native of the 19th century, but the Myst Book might well have changed hands after its original discovery before its first use. However, 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 The Book of Atrus. Since the Stranger is able to read Atrus's 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, which would be a rather unusual and highly unlikely skill for a Spanish Mexican or Amerindian native of the time period to have (almost all English speaking humans dwelled on the eastern side of the continent in the early 1800's, and the West was primarily undiscovered country).
And so, just as another possible theory as to the identity of the Stranger, it is possible that he or she may have been a lost or intentionally separated member of the Corps of Discovery that accompanied Lewis and Clark on their famous expedition to the state of present-day Oregon in December of 1805, the only English speaking humans that could have likely been found in the western part of the country during this time, which just so happens to coincide close enough with the time period in which the events of the first Myst game take place. Following this theory, it is safe to say that the Stranger was likely a U.S. Army soldier volunteer (since the expedition was military led) who left the expedition under unknown circumstances while on its way back to St. Louis, Missouri either in late March or early April of 1806 and would travel South either intentionally or by accident, eventually entering present-day New Mexico and stumbling across the Myst Linking Book in December later that year. If the Stranger was traveling by foot, it would make sense that it would have likely taken him or her over eight months to reach New Mexico from the northern part of the country for obvious reasons.
In addition, and just as an interesting observation, if the Stranger was a member of the Lewis and Clark expedition, it is somehow fitting that he or she was a part of the Corps of Discovery, since discovery was one of the expedition's primary objectives (and quite possibly the most important), and for the Myst series as a whole, is easily the central theme that dominates all others.
In the Strategy Guide
The only official definite backstory about the Myst protagonist comes from Rick Barba's Myst: Prima's Official Strategy Guide - The Stranger is a young adult photographer from the present day and, looking for books on photography, one day walks to the San Francisco public library. He finds an old book which describes with an extremely detailed island, and features a living image of that island at the end; by mistake he touches with his hand the window, and goes through the fissure into Myst. Armed only with a camera, he then progresses through the Ages of the Myst series, updating important notes in his journal, taking photos of important information for back home, and solving all of Myst's puzzles to save the day.
The Guide was written before further backstory of the Myst was elaborated, and although it is the only "official" word about the Stranger, it contradicts the later timeline. Since the Prima Strategy Guide of Myst III: Exile was released, the narrative portion of the guide was written out, thereby writing out the photographer altogether.
Other things that we can infer are:
- He is good at mathematics, as he estimated how many possible combinations there are for the Color Marble Puzzle.
- The Stranger does not know Atrus before Myst, or if he does, Atrus does not recognize him
- The Stranger is human, as the Riven star fissure leads to earth and therefore earth is where the Myst book eventually ends up. The Stranger could be Rivenese, as he could have been thrown into the Fissure by Gehn or simply fell in during the opening. However, the Rivenese NPCs speak a language unfamiliar to the player, though it is possible that the Stranger may understand it, as comprehension of the language is not necessary to complete the game.
- The Stranger is fairly smart, since he has solved all the puzzles that Atrus puts out to guard the books.
- The Stranger apparently is an exceptional photographer, as indicated in the strategy guides. He always brings plenty of film with him, and takes significant photos for the "public back home". According to the Stranger, he admits that "if I couldn't document this place, nobody would believe it."
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 touched its Linking Panel and Linked to Myst. His explorations led him 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 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 became involved in a grievance of a man named Saavedro. Saavedro, wronged by Atrus's 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: End of Ages is almost certainly 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 been dead long before either game took place. In Myst V, Atrus refers to the player character as "my old friend," but the Myst V strategy guide explains this as being a result of Atrus's age and senility. However, if Atrus is indeed still alive after having lived for so long, then it is not unreasonable to assume that the Stranger could be too. One needs to take into account that Atrus is part D'ni and even if the Stranger was Rivenese, he/she would still have died long before the events of End of Ages. There is some speculation that the player character of Myst V may be none other than Dr. Richard A. Watson, a leading member of the defunct D'ni Restoration Council.
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. This is somewhat supported in the games, and most notably the title of Uru, which can be translated as "You are you."
However, as the Myst story evolved, fans became curious about the anonymous player character. Atrus's 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.
However, the strategy guides have printed mistaken information before. Atrus calling the Myst V character 'old friend' may hint that he is a relative of the original Stranger (possibly with a family resemblance), and Atrus kept contact with his friend's subsequent lineage.