The Magistrate

The Magistrate was an incarnation of the Master who became the Magistrate of the Citadel on Gallifrey.

Biography
The Magistrate was the Doctor's oldest friend and went to the Academy with him. During his time at the Academy the Magistrate became the highest ranking lawman on Gallifrey. Following graduation from the Time Lord Academy, the Magistrate pursued a career as Magistrate for the High Council. (PDA: The Infinity Doctors)

Personality
When starting out as Magistrate, the Magistrate had devotion to justice and discipline, however this caused an obsession with order and the Magistrate became a much darker person who enjoyed causing trouble for other Time Lords. (PDA: The Infinity Doctors)