Arran

 was a Shift. He captured the First Doctor and Ian Chesterton and accused him of guilt by association. He later imprisoned Barbara Wright and took her back to his home planet. He was drawn back to the Endeavour by Ian and Susan Foreman. The Doctor tried to convince him that because of Olivan's existence that there was another way. He didn't agree with the Doctor. He confronted Myla about what she had done in the past and ignored her ultimatum. (AUDIO: The Age of Endurance)