White Guardian

The White Guardian or The Spirit of Light in Time was an anthropomorphic personification who embodies and personifies both order and good in the same way that his opposite, the Black Guardian, represents evil and chaos.

History
The White Guardian sent the Doctor and Romana on a quest to find the six segments of the Key to Time, transmuted into a variety of forms and scattered across time and space. He warned them of the Black Guardian who would try to prevent the assembly of the Key. (DW: The Ribos Operation)

The White Guardian and the Black Guardian offered Enlightenment, symbolized by a crystal of unknown powers and great value, as a prize to the winner of a space race undertaken by Eternals. When Turlough threw the crystal at the Black Guardian, who vanished in a burst of flames, the White Guardian explained that the Black Guardian would exist as long as he did, until neither were needed any longer. (DW: Enlightenment)

The White Guardian later sent the Doctor, Ace and Benny on a quest to find the six segments of the Key to Time, scattered across the Doctor's personal timeline when he last disassembled it. (DWM: Time & Time Again)

Behind the Scenes

 * The Woman resembles the White Guardian, though it is unknown if she is a new incarnation or if this is a coincidence or homage: she dressed all in white, only appeared to a few people, and indirectly tried to help the Doctor without his knowledge. Russell T Davies called attention to her costume and called it a clue on the community.