Shadowheart does behave a bit like a bipolar girl who is friends with a friend of mine. Quite a lot of push pull to her personality. I guess it is just some sort of consequence of religious brainwashing. Maybe it will make sense once we know more about her. For roleplaying reasons my mage cannot tolerate someone like that in his camp. Too much potential danger. As a romance option? Lae'zel is not coming from a society where such relationships are important, Shadowheart is untrustworthy, Wyll is attached to a Cambion and Gale is heartbroken about Mysta.
Shar seems to have a thing for luring the evil-curious, co-dependent, and vulnerable sorts into her worship. Then there's the types that know precisely what sort of toxic evil she's capable of and go in with eyes wide open.