Cass (Under the Lake)

Cass was second in command of the Drum, an underwater mining facility in Scotland. She was also deaf, and used her good friend Tim Lunn to translate her sign language.

When commanding officer Jonathan Moran was killed, Richard Pritchard took charge against Cass's wishes. She led the remaining crew to the base's Faraday cage, where they would be safe from the ghost leader Prentis.

After Pritchard's death, Cass was recognized as the leader of the team. When the Twelfth Doctor arrived, he hatched a plan to trap the ghosts. However, Cass and Alice O'Donnell stayed back to man the controls. Due to her sign language, Cass was able to understand the silent words that the ghosts kept repeating in unison.

When the Doctor decided to go back in time to before the flood to solve the ghost mystery, Cass, Lunn and Clara became trapped behind the flood doors. Case expressed her doubt the Doctor would rescue them only to stare outside in horror at the sight of a ghostly Doctor.(TV: Under the Lake)

Behind the scenes

 * Cass is the first deaf character to appear in Doctor Who. Sophie Stone is not the first deaf actor to appear on the show, however.