This is all speculation, as I doubt there will be anything new from her in future DLC... buuuut...
I'm not buying how sweet she is, though I really like the character. She is a worshipper of Molag Bal (or was) to the point of going through a horrific and degrading ritual to become a pure vampire, and still considers it a gift if you ask her about a cure before finishing the MQ.
She is also somewhere around
She is also really eager to turn you into a Vampire, or see you turned into one. Or at least shows more reluctance over a soul trap spell. I also noted just how much she grows to like the PC, and how protective she gets, to the point of making out to her father that protecting the PC is a big part of her betrayal. And it didn't escape me that Hakon made a remark about her keeping you as a pet, as though it wasn't a suprise or new, and how she didn't really argue.
Frankly, i'm half wondering if isn't just a hell of a chessmaster, and if the PC isn't just her pet in her eyes.
An alternative way to look at it is that she is pretty much you're girlfriend if you stay together to adventure. I can see why people wanted a marriage option, there really are vibes to the back-and-forths between her and the PC, and her comments. I wonder if Bethesda didn't include the option more because she is a vampire, and an ancient one at that, who may not want any part of a marriage before Mara.
Side question: can she die after the MQ? Does she stay essential? Because i'm wonderign if she won't turn up in a future TES game, at least in a history book.