- You may be looking for the human whose form she used.
An Arcateenian who went by the name Mary had been travelling with her prison guard with a transporter. In 1812, she killed him and arrived in Cardiff. She took over the body of a human prostitute named Mary, and took on her identity.
To sustain herself, "Mary" fed off human hearts, starting with the soldier that had chased the original Mary. The transporter was left behind and the city of Cardiff grew over it over the centuries.
Torchwood Three later found the transporter. Mary pursued a relationship with Toshiko Sato, intending to use her to get the transporter back. She gave Tosh an Arcateenian telepathy pendant and taught her how to use it. She continued to gain Toshiko's trust, eventually seducing her. Inside the Torchwood Hub, she had Toshiko at knifepoint demanding the transporter back. Jack Harkness tricked Mary, and killed her by transporting her to the centre of the sun. (TV: Greeks Bearing Gifts)