Mary (Greeks Bearing Gifts)

Mary was a prostitute on Earth.

Biography
While running from one of her potential johns, a soldier, the prostite named Mary found an alien (later to be discovered as an Arcateenian) in the woods. The alien took over Mary, presumably killing her, and then killed the john who had chased after Mary. While the alien possessed Mary's body, she didn't appear to age and could heal from gun shots and other wounds.

Mary developed a relationship with Toshiko Sato in order to infiltrate Torchwood 3. After she did, Jack Harkness transported her to the centre of the sun, killing her. (TW: Greeks Bearing Gifts)