She had a brother "Lucern". She was a Third Trierarch to the Varax Citrine.
As all Varaxils, she and her brother swore an oath to round up individuals who could channel Odic energy. The two crash-landed on Vetter's Tor around the 1650s. By that time, using the WitchStar they had already imprisoned many such individuals, including a flux imp and an Erisi.
When the two Varaxils were discovered in Morris's barn in their natural form, they killed Morris, his wife and the local priest, as well as the village squire's 17-year-old children, whose appearance they assumed using a transmutation matrix. Due to a fault in the matrix, they could not change their appearance any further.
Using the appearance and memories of Lucern and Finicia Portillon, they continued searching for humans with Odic potential. John Kincaid prevented them from killing one of them, Agnes Bates. Kincaid's meddling with the WitchStar caused all the accumulated Odic energy to be absorbed by Agnes, turning her into a bloodthirsty witch and making the WitchStar powerless against her.
After the Eighth Doctor subdued Agnes, the Varaxils thought that she was buried in a well. As they could not kill the witch Agnes had become, they swore to wait for a possibility to finish what they saw as their sacred mission. They did not know that Agnes Bates was freed by the Doctor and only the residual Odic wave form was buried in the well. The WitchStar was also taken away from them by Squire Claude Portillon who never trusted "his children" again.
For 350 years, they had been waiting near the well, unable to regain the WitchStar, until the witch was released from her confinement by labourers in the 2000s. Still unable to fight her, they took refuge in the TARDIS after being rescued by Mary Shelley. For Mary and the Doctor this was the first meeting with the siblings, who introduced themselves as children of Aleister Portillon, a distant indirect descendant of Claude Portillon.
Investigating the origins of the witch, the Doctor took them back to the 17th century to the time right before the WitchStar accident. The "Lucern" and "Finicia" from the future managed to remain almost unseen by the village dwellers. They were able to return back to the 21st century by using the fast return switch. At the moment, Mary was in the TARDIS, but the Doctor was left stranded in the 17th century. The siblings then stole the WitchStar from Aleister Portillon and, armed with it, they almost succeeded in killing the witch, but Mary found a way to stop them by touching the WitchStar on the Lokic containment web, inadvertently killing them in the explosion of Odic and Lokic energies. (AUDIO: The Witch From the Well)