Mary (Greeks Bearing Gifts)

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

While running from one of her lovers, Mary found a Butterfly Person 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. (TW: Greeks Bearing Gifts)

"Mary", the identity assumed by the alien, developed a relationship with Toshiko Sato in order to infiltrate Torchwood 3.