Susan Foreman was the
granddaughter of
The Doctor. She was played by Carole Ann Ford, and was featured on the show from the beginning (
An Unearthly Child) to the end of
The Dalek Invasion Of Earth.
Susan, though officially The Doctor's granddaughter, may not be offically a Time Lord. When she leaves (Or, rather, is left behind by) The Doctor she seems to be planning on getting involved with a human who was fighting against the Daleks. It is possible that she is from Gallifrey but is not actually able to regenerate - It has been suggested that The Doctor visted the past of his own world, and may have taken her with him when he left the planet, knowing what was going to happen he would have chosen to save her. Further support of this theory comes from that fact that she wasn't very good at covering up her advanced knowledge and first hand experience of history, which would be something a proper Time Lord would be expected to know how to do.
Despite having scientific knowledge superior to that of 1963 Earth and a knowledge of future history Susan still acted like a young girl most of the time. It's possible she was a few years older than her claimed age, yet had not matured well under the over-protective wing of The Doctor. She also has dormant psychic abilities, which are heightened by proximity to the Sense Sphere, home of The Sensorites.
Susan's more useful moments have included using her psychic abilities to communicate with the Sensorites and placating her crotchety and grumpy grandfather. However, with collapsing a pile of girders of the TARDIS, screaming a lot, twisting her ankle and generally being a pain she cancels it all out. She met the Daleks twice in her original travels, and again a third time when she was brought to Gallifrey with a time scoop in The Five Doctors. When we meet her this time she seems to be quite mature, though also very boring, sitting in the TARDIS for half the story, and eventually returned to Earth by The First Doctor (she twists her ankle).
I was born in another time, another world... - Susan; An Unearthly Child