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. Lunn closed his eyes in fear, but the ghost then dropped the wrench and went away.

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 walking across the lakebed. (TV: Under the Lake)

After the threat was ended, Lunn, Cass and Bennett were the only survivors of the base personnel. Looking in on O'Donnell trapped in the Faraday cage with the other ghosts, Bennett had Lunn translate a message from him to Cass that revealed that Lunn was in love with Cass and how Bennett wished he'd told O'Donnell before she'd died that he loved her. Cass is shocked by Lunn's confession and he quickly apologizes before she shocks him by passionately kissing him. (TV: Before the Flood)