The Other

The Other, Rassilon and Omega, were the founders of Time Lord society. The birth and origin of the Other would remain a mystery for later Time Lords.

Life
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.

As Rassilon's rule moved closer to despotism and Gallifrey's borders were sealed, the Other attempted to escape the planet by hurling himself into the Looms to await his eventual reconstitution. (PROSE: Lungbarrow)

Influence on the Doctor
Accounts differ as to results of this. Some imply that he was re-Loomed some ten million years later to the House of Lungbarrow as the Time Lord who would become known as the First Doctor (PROSE: Lungbarrow). Without realising it, the Doctor would retain memories of his earlier self. This showed when, as the Fifth Doctor, he met Patience. (PROSE: Cold Fusion). 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)

At some point, Patience was married to a Doctor who resembled (PROSE: Cold Fusion) one of the faces seen during the Fourth Doctor's mindbending contest with Morbius on the planet Karn. (TV: The Brain of Morbius)

The Watch raided their home and demanded that their daughter-in-law's pregnancy be aborted. A man resembling the First Doctor arrived and intended to get Patience and her granddaughter (who had already been born) to safety. (PROSE: Cold Fusion)

Influence on Gallifrey
Scholars would later call documents concerning Rassilon, Omega and the Other the ROO texts. (PROSE: Goth Opera, PROSE: The Infinity Doctors)

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

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 REF: The Gallifrey Chronicles, the Other had a detailed knowledge of Gallifreyan history and imparted some of that knowledge to Rassilon. The Other first became known to Rassilon in the weeks before Omega destroyed the star Qqaba to bring Gallifrey its power over time.