Patrina Velikovna
Unlike her brother, Patrina has no direct entry in the Appendix of Curse of Strahd. Her information is scattered around the book, but key points are summarized here.
Patrina's Past
It is Kasimir's belief that Patrina was a victim, corrupted by Strahd.
Convinced that she was the concubine of the devil Strahd, Kasimir and his fellow dusk elves stoned Patrina to death.
Page 232, Kasimir Velikov
However, it was Patrina who approached Strahd.
When a dusk elf named Patrina Velikovna came knocking on Strahd's door ... Rahadin was suspicious of her motives. ... She told [Strahd] of a vault of forbidden lore called the Amber Temple ...
Page 237, Rahadin
In life ... [she] was nearly a match for Strahd's powers. She felt a great bond with him and asked to solemnize that bond in a dark marriage.
Page 89, Crypt 21
Patrina's Goals
Patrina wants to be revived so that she can become a powerful vampire herself.
Kasimir has no inkling that Patrina is using him for exactly that purpose, and that her ultimate goal is to become as powerful a vampire as Strahd.
Page 233, Kasimir Velikov
If revived, she acts innocent.
If the characters have her spellbook, she kindly asks them to give it back to her so that she can prepare her long-forgotten spells and help destroy Strahd (a lie). ... she repays their kindness by learning as much about them as possible before pursuing her own goals.
Page 90
Once her strength returns, she seeks Strahd once more.
... whereupon she travels to Castle Ravenloft and attempts to return to Strahd, seeking to become his vampire bride at last.
Page 196, Kasimir's Dark Gift
She still cherishes her brother, and desires revenge on Rahadin for mutilating their people.
... setting [the characters] on a path to killing Rahadin, who has long opposed her marriage to Strahd.
Unfortunately for Patrina, Strahd has moved on.
Given the chance, he turns her into a vampire spawn and puts her back in her crypt - a fate she would do everything in her power to prevent.
She would not allow herself to be enslaved, but neither would she help the characters defeat Strahd for the liberation of Barovia. Patrina is evil, and would seek vampirism on her own if necessary.
Patrina also likely has no awareness of Tatyana/Ireena, and does not know that Strahd would discard Patrina in an instant if it meant choosing between her and Tatyana.