Tim Lunn

Tim Lunn was Cass's sign language interpreter aboard the Drum, an underwater mining facility in Scotland.

When their commanding officer Jonathan Moran was killed and transformed into a ghost by Prentis, Cass took charge and led the crew to the base's Faraday cage. When the Twelfth Doctor arrived, he hatched a plan to trap the ghosts that involved Lunn leading them to Mason Bennett. However, instead of staying together, the ghost of Richard Pritchard continued to follow Lunn. After cornering his victim, the ghost picked up a wrench, ready to finish Lunn off, but went away at the last moment.

When the Doctor decided to go back in time before the flood to figure out the mystery of the ghosts, Lunn became trapped with Cass and Clara Oswald behind one of the flood doors. The three then retreated into the dining area, where they caught sight of a ghostly Doctor floating in the water. (TV: Under the Lake)