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)

It is possible that the White Guardian has appeared in the Christmas special though in a new female form. When Wilff gose to a church he notices a picture in the stain glass window of the Tardis. A middle aged woman appeares then and explains a legened that came up from an encounter with the Doctor before mysteriously vanashing. The women later appeares several times to Wilff explaining that something dangerous is coming and that the Doctor can still be saved if Wilff dose what is nessasary on not tell the Doctor of their meating. The fact the women appeared only to Wilff, dressed in white with a white background, showed concern for the Doctor and offered indirect help to Will makes it possible that she is a new manifestaion of the Guardian.