Mary (Greeks Bearing Gifts)

Mary was a prostitute on Earth.

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

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