Lytalia was a female Phiadoran who was saved by Turlough from a terrible fall. He became attracted to her, because of their slender bodies. Using a touch of his forehead, she could speak English.
Upon arriving on her species' camp, she introduced them to the princess Nareena. She gave the rest of her race the power to speak the language.
When heading for the citadel, Turlough was disappointed to note that she was forced to go in a group in front that would scout for any dangers. She realised his disappointment and promised that she would return when her job was over and join him.
When Turlough returned from his journey into the citadel, she was impressed by what she'd heard of his actions during the mission.
After the saucers she had been flying had been shot down, she had landed in some undergrowth, where she managed to fend off a snarling creature. The Fifth Doctor and Turlough found her and they worked together to find the British Fleet.
When on board the British ship, Draco, she was viciously attacked by a Vrall creature that had entered the craft unnoticed. However, it was later revealed that Lytalia had actually been the host to a Vrall since before she made contact with the expedition, and her entire 'relationship' with Turlough was just part of an elaborate deception to allow the Vrall to escape the park. (PROSE: Imperial Moon)