The Whisper

 was a vigilante on New Vegas, who was created when the dying Lillian Marsh, having been thrown out of the New Vegas dome, merged with a Star Marshal. James McNeil found what remained of his daughter, and when the creature asked for instructions, he gave it the personality of a vigilante, and sent it after Cyrus Wolfsbane, who was responsible for throwing Lillian out in the first place.

He attacked the cafe where Rose Tyler worked and killed most of the customers. After Rose and the Ninth Doctor faked a robbery to attract the Whisper's attention, but the Whisper instead killed a street artist, and outran them. Due to the Whisper's defective programming, believing that everyone was guilty of something, he punished people for minor crimes, such as street art, with death, draining the life energy of those he killed to stay alive. He eventually decided to serve justice by shutting down the containment dome. At the atmospheric control centre, the Whisper confronted Wolfsbane and his men, and began killing them, including Daisy Hewitt. When the Whisper began altering the dome's generators, McNiell and the Doctor managed to persuade him to stop, with Mcneill apologising for creating it. However, Wolfbane stabbed the Whisper, fatally wounding it. The Whisper died in McNeill's arms, and what was left of Lillian said goodbye to him. (AUDIO: Night of the Whisper)