The Guardian

The Guardian was a Time Lord and a longtime friend of The Doctor. Before he took the name "the Guardian", he was known as The Brigadier and served as one of Gallifrey's foremost military commanders. He fought in the Time War, participating in many battles, fighting alongside the Doctor in many of them, including the Fall of Arcadia, where he was severly injured, resulting in his regeneration.

He was present in the final days of the war when the Doctor rejected his name and became The Renegade. When it was discovered that Rassilon and the High Council were planning on initiating the Ultimate Sanction, both the Brigadier and the Renegade opposed him. The Brigadier planned on challenging Rassilon for the office of Lord President and assuming leadership of Gallifrey. However, The Gallifrey Defense Fleet was decimated by the Dalek Battle Fleet, thus leaving the planet open for invasion by the Daleks, spurning Rassilon to begin the Ultimate Sanction. In order to prevent this, the Renegade used the Moment to destroy Gallifrey, the Daleks, and the Time Lords.

However, the Brigadier managed to escape. Bringing the Renegade aboard his own badly damaged TARDIS, the Brigadier forcibly separated the Renegade and the Doctor by channeling consecrated time vortex energy on him to separate the true Doctor and his dark side. He then abandoned the Renegade and told the Doctor to flee from this time. The Brigadier then abandoned his title and rechristened himself as "the Guardian" and time-locked the entire Time War. He then made it his mission to stand guard over the Time War to ensure that the time-lock could never be broken.