I think the heroine is gonna be an ally if she continues like this. She doesn't seems like a bad person, if a bit of the bakarina variety.
Nah, with how she acted in her intoduction with the "fake vs real" attitude it feels a lot like the typical bad heroine. Not explicitly evil, but at the very least selfish enough to be the antagonist.
About the best turn-out I can think of for her would be that she sees all of Cain's accomplishments and goes after him aggressively, which triggers Diana's secret villainess side as she gets protective of him and that causes issues. But I assume this is just gonna be a normal "she meets the other targets at the festival and that's when the wrenches start getting thrown into the plot" scenario.