White Guardian

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

Biography
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. Assembling them was said to restore balance and order by freezing all time. He warned them of the Black Guardian who would try to prevent the assembly of the Key. (DW: The Ribos Operation) Together with the Black Guardian and four others, he was part of the Six-Fold God known as the Guardians of Time.

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. 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)

On another quest to find the Key to Time, The Doctor and Amy found him in 9th century sudan, cramed in 'a measly number of dimentions enough to count on one hand, a human hand' (as he said), posing as the Legate of the Caliph, the embodiment of law in that era of time, collecting tax and punishing tax evadors, Lord Cassim to name one. He was only there after he followed The TARDIS there, as that was orders way of finding the fifth segment of the key to time. He and his adversairy couldn't trust each other to carry out the Doctors orders, watched each of as they worked. When the Doctor left the Sudan, he was stranded there waiting for the universe to end. (BFA: The Key 2 Time - The Destroyer of Delights).  He remained there for so long he foirgot who he was, however Zara found him and took him to Atrios, where he became the scientific adviser, Proffesor Lydel, for President Astra's mission to The Chaos Pool.

Thanks to Amy and the Doctor arriving, he became in contact with the segments of the key and rejeuvinated back to his former self, and therefore tried to take the key from The Doctor when he assembled all of it, even going as far as using the Teuthoidian's army which happened to be on Planet Chaos. He failed and was returned to the howling void and his etarnal struggle with the Black Guardian by The Grace. (BFA: The Key 2 Time - The Chaos Pool)

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

 * When Part One of The End of Time aired, it was speculated that The Woman was the White Guardian. 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. However, most consider her to be a Time Lady due to her appearances in the second part.