White Guardian
The White Guardian is an anthropomorphic personification who embodies both Light and Order. He opposes the Black Guardian.
History
The White Guardian set 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 need 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 and Time Again)