Is she someone's secret daughter from an incestuous bond? Ignore that, that's just my theory.
Here's the real deal reported by TV Line:
The next handful of episodes will gradually remove the proverbial bag from the mysterious Clarissa’s face. “We’ll learn how it is that she’s rooted to this castle, who it is that she’s kind of related to, and we’ll learn a back story that even she doesn’t know — and that backstory will anger her,” says EP Laurie McCarthy. As for her protection of Mary, McCarthy says, “It’s a little bit like having the Mafia as your best friend. Her moral compass is not quite set at the right angle and when she perceives a slight, she reacts pretty violently.”
What's your other theory on Clarissa's past? Hit up the comments below!