Officially she was a scientist who had a physical immunity to pain (this is a real genetic mutation, apparently) specializing in the analysis and study of genetics, particularly the equine genome. Unofficially she was a complete and total sadist with a barely tenuous grip on reality* who harvested half of her samples from unwilling victims. She's with whoever we're supposed to be going after (either the Dark Forces or the Cult of the Eclipse)
*Wow, HT made a character suffering from insanity. Who could've possibly seen THAT coming?