Captain Mike Yates (Richard Franklin)
Captain Yates - known to the Doctor and Jo as Mike - was the Brigadier's easy-going right hand man. As such he helped to repel a number of alien invasions, and had various encounters with The Master.
When Mike was under cover, investigating Global Chemicals, he was hypnotised by the megalomaniac computer, BOSS, and turned against the Doctor. His loyalty was again tested when his idealism made him betray his UNIT colleagues for the scientists planning Operation Golden Age.
Later, whilst recovering, he discovered that a Buddhist retreat was being used by giant Metabilis III spiders as an invasion channel, and somewhat redeemed himself.