The Magistrate

In another universe, the Magistrate of the Citadel was a Time Lord who was the most senior member of the Prydonian Chapter and the President's most trusted adviser.

He was also the Doctor's oldest friend. They attended the Academy together, and though they'd had their differences in the past, Larna saw that they clearly loved each other. They were the only members of the High Council less than two thousand years old.

He worked with the Infinity Chambers every day. (PROSE: The Infinity Doctors)

Appearance
He was of medium height, with a sallow face and a small, pointed beard. However, his most remarkable physical feature were his dark, burning eyes. He wore black robes. (PROSE: The Infinity Doctors)

Behind the scenes

 * The Magistrate is obviously intended to be an incarnation of the Master, though this is never stated in-text.
 * The War King in The Taking of Planet 5 is intended to be the Magistrate; he remembers to Homunculette an exchange between the Doctor and Sontar seen in The Infinity Doctors. The Book of the War also later hinted that the War King was the Master and War Chief.