A Roman legionary was a guard at the gates of Pompeii. He was incapacitated by Aglae, but came to his senses and tracked Aglae and Mel down, returning them to the prison in Pompeii.
During the volcanic eruption, he found Mel after she had escaped from prison, and took her to a shelter for safety. When she left to look for the TARDIS, he remained behind. (AUDIO: The Fires of Vulcan)