Larna

In one universe, Larna was one of the Doctor's best students at the Academy when he taught on Gallifrey.

Biography
Larna developed a relationship with the Doctor during her time at the Academy and commited suicide with a knife to try and stop the Doctor leaving for the Station. She was resurrected by Omega at the Doctor's request. Larna set off to tidy up the “side-effects” from an incident which caused history to change. (PROSE: The Infinity Doctors)

Larna once travelled back in time to involve herself in Gallifrey's past. She travelled to the Shoal, where she began monitoring a cicatrix in time and sent information back to Gallifrey. During this time she met Marnal, a Time Lord who had been sent to destroy the Shoal.

Larna later met the Doctor's parents, Ulysses and Penelope Gate and told them that she was from their planet's future. She gave the High Council a prophecy that told of Gallifrey falling after a number of attacks from different species. A single Time Lord would have a hand in all of those events and he would find the Lost Scrolls of Rassilon and lead Gallifrey from darkness. (PROSE: The Gallifrey Chronicles)

Larna worked for the Doctor's father in Berkeley, San Francisco in 2002 on the Advanced Research Project. (PROSE: Unnatural History)

It is not clear when or how she travelled from her original universe to the Doctor's, or how much the two universes held in common.

Behind the scenes
Larna's name appears to be a reference to "Lady Larn", the name given to Susan before her travels with the Doctor in Eric Saward's story Birth of a Renegade, published in the Radio Times Doctor Who 20th Anniversary Special.