Eu não estou totalmente ciente do que é considerado canônico aceito no Who'versse, mas tudo o que eu posso encontrar corrobora que é o Guardião Branco que define o médico originalmente.
De uma transcrição do programa (Admitidamente, o Guardião poderia estar mentindo):
GUARDIAN: I am the White Guardian. In order to maintain the universal balance, there is also a Black Guardian, and he also requires the Key to Time, but for a different purpose. An evil purpose. He must not get it. Doctor, at all costs, you must prevent that.
Do guia do Doctor Who :
The Doctor and K-9 Mark 2 are sent on a mission by the immensely powerful White Guardian. The six segments of the Key to Time, currently disguised and scattered throughout the cosmos, need to be brought together. Once assembled, the Key will allow the Guardian to restore balance and good to a universe that is descending into evil and chaos. It will also thwart the plans of the fearsome Black Guardian!
De TvTropes :
A godlike guy in a casual white suit, the White Guardian, interrupts the Doctor's puttering around in the TARDIS to give him a quest: find all six segments of the Key To Time. Why and how and what the thing does isn't explained just yet, but the Black Guardian apparently wants it too.
De O antro de geek :
If you really want to break out the big guns, you can argue that Season 16, during Tom Baker's tenure, was the longest story in the show's history, running for twenty-six episodes.
The Doctor is enlisted by the White Guardian to find the six separate segments of the object through time and space, and assigned a Time Lady companion, Romana, to help.
Provavelmente uma das corroborações mais strongs é de um livro intitulado Guia do Doctor Who Program por Jean Marc Lofficier (não consegui encontrar o trecho do episódio real no google books, isso é de uma sinopse neste site ):
SEASON SIXTEEN
The Ribos Operation 9/2/78 9/23/78 4
Sent to seek out the Key To Time by the White Guardian, the Doctor and his fellow Time Lord Romana, find themselves sucked into a botched mineral scam on the planet Ribos.