Heather Threadstone

Heather Threadstone was the daughter of Professor Threadstone and a scientist herself, like her father. Like her father, she was a friend of the Sixth Doctor, and accompanied him during the investigation of the Dominus Institute in 2016. (AUDIO: Vampire of the Mind)

Behind the scenes
While she has only appeared once, she is nonetheless called a new companion of the Sixth Doctor in the behind the scenes interviews on the Vampire of the Mind CD.