Sentreal

Sentreal was a representative of his home galaxy at the first meeting of the Galactic Council, when he allied himself with the Daleks in their plan to conquer the Solar System. (DW: Mission to the Unknown) He did not appear at the seventh meeting, his place taken by Zephon. He seems to have returned by the time of the final meeting however, perhaps because Zephon had been exterminated. After he was betrayed by the Daleks, the Doctor convinced him to go back to his own people and warn them about the Daleks' invasion plans.

Sentreal, a strange half creature, half vegetable being, was all black, apart from two burning, bright eyes in his cylindrical head. His body looked as though it consisted of several open cones stacked on top of each other.

Behind the Scenes
The actor who played Sentreal did not receive a credit, but Johnny Clayton and Len Russell played 'Alien Delegates', while Pat Gorman played a 'Planetarian'. It remains unclear, to date, which of them played Sentreal.