Katya lived in a homestead with her mother and near her boyfriend Christopher. She was kidnapped by Count Bram who suffered from mental illness and mistook her for his deceased wife Annabelle.
The whole point of the island quest is to rescue Katya from Count Bram. When you first encounter her, she is trapped in a massive cage dangling from the ceiling. When you tell her about Count Bram trying to reverse the vampirism process, she gives you the location of the book Root Causes (which is optional). After you've perfected the anti-vampirism serum, you must go to the bottom right of the room she is in and use the skull cane to hook and drag the cage key from within storeroom. After unlocking her cage, Count Bram arrives and, still mistaking her for his wife, desperately attempts to stop her. A chase ensues as Count Bram chases you and Katya around the rooftops of the castle. At last he transforms into a bat in an attempt to catch you and you nail him with the anti-vampirism serum as he knocks you and Katya off the roof. He then reverts to his mortal form and apologizes to you and Katya for his insanity and then dies, rejoining his beloved wife in the afterlife. Christopher then arrives to "rescue" Katya and they kiss.
Katya only appears at the beginning of the bonus quest, firstly shocked that Christopher has transformed into a vampire and afterwards tackling Cactus Von Garlic to buy you some time after Christopher transforms you into a vampire.