The Other

The Other was one of the original founders of Time Lord society, along with Rassilon and Omega. The origin of the Other remained a mystery for later Time Lords.

The Other had a mysterious connection to the Time Lord known as the Doctor. According to some accounts, the Other was reincarnated through the Looms into the Doctor. (PROSE: Lungbarrow, et al.) By other accounts, the Other was the Doctor in his later life, having travelled back in time to aid in the start of Time Lord civilisation. (PROSE: The Scrolls of Rassilon, COMIC: The Lost Dimension)

Biography
One of John Smith's repressed memories told the story of a human who built a TARDIS to help the police but got lost on a jungle planet, where he taught the natives and elevated their society into a mighty civilisation. (PROSE: Human Nature)

A Time Lord calling himself the Stranger originated from a time long after Rassilon's life. Though Rassilon feared that the Stranger was planning to erase his people from history in revenge for being shunned, the Stranger was intent on guiding Time Lord civilisation away from tyranny. He travelled back in time to seven days after Rassilon had returned from the vampire wars and took power. With a pass signed by Rassilon himself, the Stranger gained access to the restricted areas of the Citadel and for the next few months offered advice to Rassilon without providing any detailed information. (PROSE: The Scrolls of Rassilon)

A stranger in the shadows chastised Rassilon when he taunted the Pythia. (PROSE: Cat's Cradle: Time's Crucible)

According to The Thousand and Second Night, someone who was neither an "angel" nor a "djinn" advised "Allah" to hide the caldera inside Earth. The Shift later claimed to have been this person. (PROSE: Head of State, Interference)

The Other was influential during the Dark Times on Gallifrey after the cult of Pythia had ended. He and the Hand of Omega shared some sort of a bond. (PROSE: Lungbarrow)

When Omega's plans to detonate the Q star were close to fruition, the Stranger explained his origins to Rassilon. The Stranger pleaded with Rassilon to make the Time Lords less powerful and less stuck in their ways, believing that in doing so the Time Lords of the future he came from would be open to new ideas. Rassilon refused to abandon his philosophies, intending for Gallifrey to become the powerful society that he had always planned.

The Stranger visited Rassilon's quarters angry about the release of the virus that gave the Gallifreyans the ability to regenerate, as it wiped out a large proportion of the population upon its release.

Shortly before the detonation of the Q star, the Stranger materialised his TT capsule on Rassilon's ship, and once again pleaded with him to alter the course of established history. Finally understanding that Rassilon would not change his mind, the Stranger left in his ship. However, respecting the Stranger's audacity, Rassilon decided to ensure Gallifreyan society would never tolerate tyranny. It also led him to write the first Law of Time. (PROSE: The Scrolls of Rassilon)

As Rassilon's rule moved closer to despotism and Gallifrey's borders were sealed, the Other took his Granddaughter to the Spaceport for safety, promising to return one day. He then attempted to escape the planet by hurling himself into the Prime Distributor of the Looms to await his eventual reconstitution. (PROSE: Lungbarrow)

According to one specific account, the Other was simply an alias used by the Eleventh Doctor during his intervention into the creation of the Type 1 TARDIS. Rassillon noted that he was acting just like the "chrono-scrollwriters of old" by refusing to give his real name. (COMIC: The Lost Dimension)

Influence on the Doctor
Accounts differed as to results of this. Some implied that he was re-Loomed some ten million years later to the House of Lungbarrow as the Time Lord who became known as the First Doctor. (PROSE: Lungbarrow) By his seventh incarnation, he seemed once more to have realised his earlier identity or else Lady Peinforte could not have threatened him with revealing it. (TV: Silver Nemesis) Cavis and Gandar hinted in their conversations at the Doctor being the Other. (PROSE: The Shadows of Avalon)

When the Seventh Doctor transformed himself into a human named John Smith, he removed almost all of his memories, but he was unable to remove the ones from before he was born. "Verity" kept these memories out of John Smith's head. (PROSE: Human Nature)

Influence on Gallifrey
Scholars later called documents concerning Rassilon, Omega and the Other the ROO texts. (PROSE: Goth Opera)

A minor Gallifreyan festival known as Otherstide was celebrated yearly in his honour. It coincided with the Doctor's naming day/Birthday. (PROSE: Lungbarrow, AUDIO: Cold Fusion)

Secret societies on Gallifrey were dedicated to the worship of Rassilon, Omega, and the Other. (AUDIO: Intervention Earth)

In a parallel universe where the Sixth Doctor led Gallifrey in the War, the Master commanded a time dreadnought named the Arcane Mystery of the Other. When he defected to the enemy, the ship was thought lost; in reality, it was being dissected in an enemy shipyard. (PROSE: The Quantum Archangel)

Other universes
In an alternate universe, the Other spoke to Omega's wife, Patience, and implied that the Fragment (the surviving portion of a collection of prophecies by ancient Gallifreyan seers) was a forgery. The Other was later present at this universe's Qqaba when Omega was lost. (PROSE: The Infinity Doctors)

Behind the scenes

 * The first intentional and direct mention of the Other appeared in Ben Aaronovitch's Target Books Remembrance of the Daleks, in the form of a brief flashback to the time of Rassilon. Andrew Cartmel and Ben Aaronovitch had already worked out a backstory to the Doctor into which the Other figured.
 * According to PROSE: The Scrolls of Rassilon, the Stranger had a detailed knowledge of Gallifreyan history and imparted some of that knowledge to Rassilon. The Stranger first became known to Rassilon in the weeks before Omega destroyed the star Qqaba to bring Gallifrey its power over time. The clear implication was that the Stranger was the Doctor, gone back in time in an attempt to make Gallifrey more open-minded.
 * The Cartmel Masterplan allegedly included a revelation that the Other was an incarnation of the Doctor. When the show was cancelled in 1989, these plans were scrapped, though they continued in the New Adventures novel line. (DWM 341)
 * When asked if the Other could be Nyarlathotep, Andrew Cartmel admitted not knowing anything about a "Nyarlathotep", the idea comes from some hinted links between him and the Doctor, notably in PROSE: The Death of Art
 * John Smith's repressed memories superficially resemble the story of Dr. Who.