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 Elliot, remained on Earth. (PROSE: Attachments)

Arriving in an alien marketplace, the Doctor and Oliver met Gravkrom-Vey, owner of the monkrah fish in the universe. The Doctor discovered the Monkrah had been wiped out in a civil war with the Manicoll, who also began dying soon after the war had ended, so the Doctor and Oliver helped Gravkrom-Vey release the last Monkrah. (PROSE: Plight of the Monkrah)

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 returned 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. (PROSE: Puppeteer)