Like the title says, I'm trying to achieve "Lustful" with Kasyrra, but I can't seem to manage. I'm either hostile or romantic every time.
What do I need to do to stay on the lustful path, and is it possible to save edit to fix it?
There's no mechanical difference right now between Lustful and Hostile so you can't really tell which of those two states you're in without checking your save file.
If you're on her Romantic path there are a couple things you can do to bump her down a notch. Attacking her in Winter City will bump you down from whatever you were at and choosing No Way after talking to Kas about Her Goals following the Temple of Sin event will bump you down from Romantic to Lustful (as you have to be Romantic to see the event in the first place).