Shadowheart is an evil cleric of Shar. As such she's not exactly likeable as she's secretive, sarcastic and clearly doesn't trust you (which are all understandable traits for a follower of Shar) but she has her good points. By playing the game you learn she's got a mission and has some kind of nefarious agenda she needs to accoplish for her cult. That said I think she's more of a VICTIM than someone who's actually evil by choice.
Personally I think there's evidence she is a former follower of Selune who was kidnapped and brainwashed by Sharrans.
Reasoning:
During your visit to the blighted village Shadowheart reacts strangely to you reading a certain piece of information about young people disappearing from the village (then dedicated to Selune...). If you press her she admits she's a follower of Shar and mentions that those disappearances seem like the work of "her people" (it later becomes clear dark justicars of Shar were active in the area, something most likely linked to Halsin's actions and his circle role in defeating the forces of the goddess of night). Also, there's an event where Shadowheart emits a strong luminosity she cannot control and seem to cause her pain. Again, if you ask her about it and suceed with your rolls she tells you it's not Sharran magic but "it's very dangerous". Lastly, if you pursue the romantic option with her Shadowheart opens up a little and tells you that she does not remember much about her past. She thinks that's so because it's apparently common practice among Sharran to erase part of the memories of their operatives to better obfuscate their motives while making them less easy to identify.
In light of all of that I think one of the possible paths that will be open with Shadowheart will be to redeem her turning her against Shar. I also think her backstory and the sheer number of hooks she brings to the story make her the most interesting NPC introduced in BG3 thus far.
