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)

"The Old Man and the Police Box"
One of John Smith's repressed memories inspired him to write down the story of a peculiar human he simply referred to as the "Old Man" in the title of the tale. The Old Man was a silver-haired inventor during the reign of Queen Victoria who built a TARDIS, having initially invented the concept of the police box to help the police, only to make more and more improvements to it until it was the familiar dimensionally-transcendental timeship.

Being a very lonely old man, he realized that his invention could take him anywhere he wished and used his TARDIS to go explore the universe. He soon got lost on a jungle planet called Gallifrey, where he taught the natives and elevated their society into a mighty civilisation, rooted in time travel and embracing the ideals of law-abiding and peace the Old Man had stood for in his home time.

Finding his protégés dreadfully boring after a while, the Old Man invented a way for them to begin life anew, and implanted them with a second heart in the hope that it "would make them happier", but these measures only served to make them live longer, not more interestingly.

Still according to Smith's tale, the Old Man "eventually" grew so bored that he stole a time machine and returned to Earth, breaking one of the laws he had created in the process. (PROSE: Human Nature)

A stranger from the future
One account stated that 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)

According to a similar account, "the Other" was simply the alias used by the Eleventh Doctor during his intervention into the creation of the Type 1 TARDIS. Rassilon noted that he was acting just like the "chrono-scrollwriters of old" by refusing to give his real name. During his prolonged, time lock-defying stay on Ancient Gallifrey, the Eleventh Doctor also advised Rassilon in technological matter, with Rassilon admiring the man's unorthodox genius but remaining wary of him. (COMIC: The Lost Dimension)

Shaping the history of Gallifrey
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)

A small man was present when Omega left to detonate Qqaba. He spoke with Omega's wife Patience and implied that the Fragment was a forgery. (PROSE: The Infinity Doctors)

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)

Re-Loomed
After the Other threw himself into the Prime Distributor, he was re-Loomed some ten million years later to the House of Lungbarrow as the Time Lord who became known as the Doctor. (PROSE: Lungbarrow) That entire generation was affected by anomalies in the Looms, resulting in several other renegades. (PROSE: The Book of the War)

By the Doctor's seventh incarnation, he had apparently realised his earlier identity. Lady Peinforte threatened to reveal it, (TV: Silver Nemesis) and Cavis and Gandar hinted at it in their conversations. (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)

Legacy
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)

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.