Companion (The Next 20 Years)

A companion of the Doctor was tricked by the Master.

In 1990, the companion told "the Doctor" that the Master had escaped from them again as they entered the TARDIS. However, she was actually speaking to the Master who was masquerading as the Doctor. The Master proclaimed that he had finally fooled the Doctor after he was left alone. (NOTVALID: The Next 20 Years)

Behind the scenes
The companion bears some resemblance to Melanie Bush but this must be a coincidence as she did not debut until 1986 three years later.