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 Lunn to translate her sign language.

When commanding officer Moran was killed, Cass took charge. She led the remaining crew to the base's Faraday cage, where they would be safe from the ghost leader Prentis. (TV: Under the Lake)

Behind the Scenes
She is the first deaf character to appear in Doctor Who, and Sophie Stone is the first deaf actress to appear on the show as well.