Councillor was a title given to certain members of the High Council, particularly those who were not Cardinals. (TV: Arc of Infinity)
Brolin was Councillor during the reign of Lord President Slann. (PROSE: Birth of a Renegade)
Hedin was Councillor during the Antimatter Crisis. (TV: Arc of Infinity)
Rath, Elar and Morin were all Councillors at the same time. (PROSE: Blood Harvest)
Flavia was Councillor before becoming Chancellor. Engin became Councillor sometime after Goth assassinated the President. (PROSE: The Eight Doctors)
Both Ragnar and Milvo sat on the Temporal Scanning Service's oversight committee and thus were a Councillor. (PROSE: World Game)
Sardon, Ratisbon and Ryoth were all Councillors and members of the Celestial Intervention Agency (PROSE: World Game, The Eight Doctors)
Delox (AUDIO: Appropriation), Romana II (PROSE: Blood Harvest, Human Nature), Wblk (PROSE: Heart of TARDIS), Verne (PROSE: The Twin Dilemma), Norval (PROSE: The Infinity Doctors), Ortan, Taskor (PROSE: World Game), Samax and Kandarl (PROSE: The Ancestor Cell) were also all Councillors at one point.