The Stranger was intended to be an anonymous entity without any history. It was implied that it was a humanoid in the original Myst games, and for their own convenience while making Uru: Ages Beyond Myst, Cyan Worlds made the "stranger" a human from Earth.

It is a person of unknown origin (it is widely believed it is an ordinary human, possibly from New Mexico) who discovered the Atrus' Myst book that fell from the Star Fissure to the Mexican desert and somehow found its way to his possession. Reading it, by mistake, the Stranger was Linked to Myst where he was stranded until he met Atrus and freed him from K'veer where he was trapped by his sons (according to the official D'ni timelines, this happened on 12 December 1806 CE). On 2 January 1807 CE, the Stranger then Linked to Riven to free Atrus' wife Catherine from Gehn. Then, Atrus let the Stranger fall into the Star Fissure in order to return home to Earth.

About ten years later (1816 CE) the Stranger met the couple again on Earth (Tomahna) and helped them to recover the Releeshahn book from Saavedro. As years progressed, the Stranger even recovered Atrus' daughter, Yeesha from her brother who kidnapped her in order to pursue his plan for revenge. Atrus refers to him simply as 'my friend'.

The Myst games and novels are represented within the fiction as taken from the journals of Catherine, Atrus' wife. In the games, his role is assumed by the player, and as such the games are seen from the Stranger's perspective.

Identity

The identity of the Stranger is one of the best-kept secrets in the Myst universe, and little is known about this person, not even whether the Stranger is male or female. It is notable that in the Myst games any dialogue is absent, so we don't know whether the game characters asked the Stranger questions about his origin or what reply he might have made. Saavedro mistook the Stranger for Atrus during the events portrayed in Myst III: Exile, suggesting that perhaps the Stranger is of similar general appearance to Atrus. However, Saavedro's initial view of the stranger occurred from behind a glass portal at a fair distance - thus still permitting broad lattitude in assuming anything about what the Stranger may look like.

The game Uru: Ages beyond Myst revealed that the Cleft, and thus the location where the Myst Linking Book was found, is in Bernalillo 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 must have been either a Spaniard (Mexico), or an Amerind native. But he could be as well some collector who bought the Myst Book from some dealer, after having changed many hands after its original finder.

Whatever the Stranger's identity as a 'historical' person of the Myst Universe and Atrus' life as his friend might be, it is most likely a simple blank, irrelevant to his participation to the history, which allows any player to play the games without having to "play" a character. Players are thereby free to approach the puzzles and plot without having to account for any backstory, special abilities or personality factors for the Stranger.