Oliver Day

Oliver Day was a human who recovered from a bout of amnesia to discover that he had spent several months travelling with the Fourth Doctor.

Biography
Oliver's amnesia was caused by an incident with an alien creature and an explosion in a house. He travelled to the scene of the explosion. In the rubble, Oliver met the monster again. It threatened to kill him, but he was saved by the Doctor, who offered to let Oliver travel with him, although his girlfriend, Chloe remained on Earth. (ST: Attachments)

After several adventures, the Doctor and Oliver arrived on the planet Gondovan, finding the investigator, Annajin Valentin, while they were investigating a being called the Puppeteer. The Puppeteer possessed Oliver, but the Doctor drove it out of Oliver and killed it. However, Oliver feared that if he returns to Earth and his girlfriend, then the residual murderous impulses left over from his experience might lead him to hurt her, choosing to remain with Annajin on Gondovan. (ST: Puppeteer)