Deborah Castle

Deborah Castle (née Gordon), also known as Debbie, was a teacher in the 20th century. She was associated with the Eighth Doctor throughout the 1980s, becoming a close friend. (PROSE: Father Time) Marnal listed Debbie as one of the Eighth Doctor's companions. (PROSE: The Gallifrey Chronicles)

Biography
In the early 1980s, Debbie once taught Miranda Dawkins and helped keep her safe from Ferran.

Debbie and the Doctor once saw the film Bill and Ted.

In 1989, Debbie spent several days in transit with the Doctor across the globe to steal the Atlantis. They used it to rescue Miranda, who was in a spaceship in Earth orbit. As a stowaway, Deborah was nevertheless one of the few Britons to fly on an American spacecraft. She was killed by a booby-trap in Miranda's room that Ferran had left for the Doctor. (PROSE: Father Time)

Behind the Scenes
According to a What If set visit published on the BBC website around the time of the novel's publication, Debbie was "played" by actress Minnie Driver.