I'm pretty open to whatever! I can't imagine it'd be a terribly nice person, but really I think we can make anything work :)
Basically what I was thinking, is my character's half demon, which would make her vulnerable to demonlords who control demons, yeah? She's free now, but if she encountered one with ill intentions and became subjected to them, she could be used as a weapon (for whatever cause - military conquest, political intrigue, bounty hunting, assassination... take your pick). Because she's only half demon it'd have to be a pretty expert demon lord to control her, and she'd exploit every opportunity to break free and take vengeance.
Then there's the possibility that because she's half human, she might inspire some small feelings of guilt and sympathy in the demon lord, which gives way to a whole host of other scenarios.
Make sense? This is just the general gist I had in mind, trying not to define it too much so as to allow other input and ideas :)