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, Colin Baker refers to her as a new companion of the Sixth Doctor in the behind the scenes interviews on the Vampire of the Mind CD. Furthermore, in the behind the scenes interviews for Quicksilver, Baker says that, "if [he] could have a third companion [he'd have] Constance Clarke, Flip Jackson, and [Heather Threadstone]".