Mary (Greeks Bearing Gifts)

Mary was a prostitute on Earth who seduced soldiers from the British Army.

Biography
While running from one of her lovers, Mary found a Arcateenian in the woods. The alien took over Mary, presumably killing her, and then killed the lover. While the alien possessed Mary's body, her body didn't age and could heal from gun shots and other wounds.

"Mary", the identity assumed by the alien, 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)