Fiona is filled with Azoth, the essence of life itself...and her character arc is largely defined by her will to survive against all odds. (Additionally, it ends with her outwitting and killing an animate skeleton- which are universal symbols of death.)
The final confrontation with Lorenzo seems to be based on "Death and the Maiden", which was a very popular motif to paint in the Renaissance. Fitting for the game, as the motif is developed from the Danse Macabre with erotic subtext.
The more the game progresses, the player might start to notice that Fiona doesn't seem to enter 'Panic Mode' as much since the beginning of the game where she's liable to panic very quickly when Debilitas finds her. Given what Fiona had to go through, and with the help of her canine companion Hewie, she's growing more accustomed to the dangers she knows she has to face in the castle, is beginning to understand why she is in the castle in the first place, and is learning to conquer her fears in order to truly escape from her predicament. Defeating her various stalkers in spite of their overt power would also give her a boost in confidence.
Debilitas' Meaningful Name may have a justification: Riccardo created him, so he was probably the one who named him; Riccardo would be smug, cruel, and pretentious enough to call a deformed, mentally stunted person "Sickness" in Gratuitous Latinand find it amusing.
Fridge Horror:
If you read the lyrics of "Endless Zero", you realize Ayla, Fiona's mother, also wanted the Azoth. Before the game starts, they were heading for the castle in the car. Fiona is 18 years old when the game occurs.
Daniella's words to Fiona just before she becomes the second pursuer are how "she's unable to taste, feel pain, or feel pleasure". She knows that she cannot taste anything due to her apathy with tasting the soup she had made, and the amount of self-abuse she inflicts such as the cuts on her hands and bashing her head against the window confirms that she struggles to feel any sense of pain. Then there's the pleasure part...how exactly does she know that she cannot feel pleasure, and what kind of pleasure exactly is she talking about? Unless...
Then you get into the fact that she's in the same castle as Riccardo, who isn't above rape and abuses her physically. Who's to say he didn't abuse her sexually as well?
