Need some help answering the old question "Why?"

[Koritai]

So, I have a question I need help answering. I'm thinking about writing a little short story, or perhaps just a story. I don't know how long it will turn out to be. The idea is pretty rough at the moment and it is bound to change, but the basis of my story is that a young woman ends up as the "companion" (She's more of his wine cellar) of the most evil man in the world who happens to be the second oldest vampire in the world. (Might not stay a vampire. Like I said, pretty rough idea at the moment.) The story is about her changing from the sweet innocent thing she used to be into this terribly evil nasty woman. What I keep getting stuck at is why the most evil man in the world would pick her? What makes her stand out? All I'm asking from you is any possible reasons you can come up with. I've had someone flame me for this sort of thing before (Not here, but elsewhere), but all I want is some brainstorming that can help me break out of this tough spot I'm stuck in. I'm not trying to steal anyone's ideas, I just want to know possibilities.

[Enigma]

Corruption. As you say, she's a sweet, innocent thing - he's seen it all, jaded to the point that nothing can really rouse his interest. By slowly corrupting her, seeing her responses, he gets to relive it again.

feel free to accept or reject anything I've posted here.

[Koritai]

This is true, and thank you so much for your idea. It makes perfect sense really. I didn't think about how boring he might get as the most evil man in the world. Well, then again, all I've gotten in making his character is "The most evil man in the world."

[Collector]

Really, I think the thing you need to do is give her a few defining traits as well as expand on your "most evil man in the world". Yes, you need to ask what makes her stand out, but you also need to ask what your villain would see in her.

If she's a nun, what makes her stand out from the other nuns? Is she more pious? Is she such a virgin that she's actually managed to have sex a negative number of times? Or perhaps her father is a man that your villain wants to hurt in some way and just so happens to do so by corrupting his daughter?

Another thing you could consider is that she comes to him, not the other way around. Perhaps she tries to change your villain for the better, but he winds up turning her into something evil. Even better, he could try forcing her to do things by telling her it's the only way she could help him change. Perhaps he could say that the heart of a child is needed for a ritual to drive the evil out of him, when in reality it's actually a ritual to summon an evil spirit into her?

There are plenty of ways you could go with this. Have fun.