If you sleep with her in the first encounter she is generally more friendly with the player and is actually kind of hurt and upset if you choose to defy her and attack her at the winter palace.
I justify this by thinking that during the first encounter the player was compromised and overwhelmed with desire by the magic of the portal Kas came out of. I refuse her at the Wayfort, but give in during the dream with her after talking. After this I see it as the character having developed enough to resist these feelings to some extent and seen how truly dangerous Kasyrra’s influence is to not want to have anything to do with her if the can help it