Trying my best not to spoil anything but based on my interaction with her, she isn't as bad as your may think. She also doesn't have her full memories. She does have a secret that has to do with the item she carries which also has something to do with the Gith. When you do Lae'zel's mission to go to the Gith camp, you'll find a reveal.
As for Shadowheart's personality, based on your decisions seems like she can be your best friend, also depends how you treat her. She does act like a cleric that always worries about your safety, which I do like.
