Mary (Greeks Bearing Gifts)

Mary was a prostitute on Earth in 1812. In that same year her body was taken over by an Arcateenian killing her in the process.

Mary (Human)
While running from one of her clients, a soldier, Mary found an alien (later to be revealed as an Arcateenian) in the woods. The alien took over Mary, presumably killing her. (TW: Greeks Bearing Gifts)

Mary (Arcateenian)
After she had taken over the human Mary's body she then killed the soldier who had chased after her. While she was possessing the human body she didn't age and could heal from gun shots and other wounds.

In 2007, Mary developed a relationship with Toshiko Sato to infiltrate Torchwood 3. After she did so, Jack Harkness transported her to the centre of the sun, killing her. (TW: Greeks Bearing Gifts)