You're asking if you can be good and still love evil, with nothing changing. It can't. Something must give, and if you won't turn evil and Kass won't turn good, it'll be the relationship.
Or the world, as your delusion keeps you from doing what you must to stop her.
That's what's so interesting about her. She doesn't seem to want to rule the world or whatnot, sure she likes corrupted things but she could just corrupt the player, but she doesn't, leaving me with two in story explanations. Either she sees you as a challenge to help grow and overcome later, which !da she's more of a competitive antagonist, or she enjoys the player beating her minions and growing stronger, and wants herself to be beaten, which doesn't make much sense. She could also perhaps want to rule alongside the player, which again, isn't truly evil I don't believe.
Anyway, I feel she is in a moral gray area, seductive yet also not too evil, she for example has moments where you can choose not to have sex, and she lets you go, and she could easily have just fucked you anyway. She is just so interesting, and I for one can't wait for the time when she gets more and more time to shine.
I feel we could both stay morally grey in game terms, she also kindly is nice to the player at times, when she, again, clearly doesn't need to be. The player knows her disguise in the Halloween thing, but she lets them go, or is again, kind.
Heh...I said a lot of rambling here didn't I? Anyway, I actually love discussing this, so feel free to debunk my points and offer rebuttals so we can further figure her out.