The Doctor's mother

An unnamed individual was the mother of the Doctor.

Biography
The Doctor said that his mother used to sing the Zagreus nursery rhyme to him. (AUDIO: Seasons of Fear) When the Doctor was just a "small child", the Doctor's mother sat by the side of his bed and told him the story of Grandfather Paradox. (PROSE: The Gallifrey Chronicles) The Eighth Doctor relived a memory from his first incarnation where his mother watched as his father held him up as a child to see the stars. (PROSE: The Eight Doctors)

Other universes
When the Doctor was young, his mother and father went on a trek in the mountains with him. The Doctor's father and mother owned a summer house on the other side of Kasterborous. (PROSE: The Infinity Doctors)

Appearance
The First Doctor remembered his mother having red hair. (PROSE: A Big Hand for the Doctor) The Eighth Doctor also remembered that his mother had long red hair, and a cut-glass voice; (PROSE: The Gallifrey Chronicles) according to him, she resembled to the third incarnation of Romana. (PROSE: The Ancestor Cell)

Behind the scenes
Russell T Davies has stated that the unnamed woman, implied to be a Time Lord, in the End of Time was meant to be the Doctor's mother. However, he has also said that who the woman is up to judgment, and she could be someone else.