Quell was the captain of the Orient Express. A former soldier, he suffered from post-traumatic stress. After Mrs Pitt and a chef were killed, the Twelfth Doctor confronted Quell about taking action. He refused on the basis that he had already followed the protocols for accidental death, not believing the Doctor's claims that the Foretold, a mythical mummy, was responsible.
When one of his guards was killed, he accepted the Doctor's theory. The scientists about the Express discovered that it was targeting the weak, and the Doctor informed Quell that his mental issues made him the next target after Professor Moorhouse. When the Foretold came for him, he fired upon it, claiming he would be a poor soldier not to do so. As it moved to kill him, he thanked the Doctor for "waking him up" and considered his demise a good death. (TV: Mummy on the Orient Express)