Brolin

 was a Councillor on the High Council of Time Lords during the reign of Lord President Slann, and possibly that of his predecessor, Pundat the Third. His adopted daughter, Lady Larn, was secretly the last living descendant of Rassilon. Student revolutionaries hoped to use her as a figurehead in a new government, but after two coup attempts failed, Larn hid aboard a stolen TARDIS. She was discovered by the Doctor, who had stolen the TARDIS; she called him "grandfather" out of affection. (RT: Birth of a Renegade)

Behind the scenes
This account of Susan's origins is not reflected in any other source. It contradicts the original intent of the programme makers when Susan was a regular character on the series. It is also contradicted by the later novel Lungbarrow.