I find her so unpleasant and annoying, I don't care about her 'mysterious past'. I kill her and that takes care of that.
I generally killed Sebille in DOS2 because anyone who wants to murder you for merely looking at them probably needs to be put out of their misery.
HAHahahahahaha!
Can we bro-fist on that?
Sybille yeah that was her name.
Killed her too once just for that stunt.
But I reloaded afterwards cause I was still too curious to see what her Edgelording was all about. ^_^
And because I wanted a Rogue in my party.
I like all the companions in their own way, but that doesn't stop me thinking that some of them are actual grade A morons (wyll and gale im looking at you)
What? Why them? Those are the most "sane" people there!! ^_^
Wyll is the literal Hero archetype and he shares my great hate for Goblins. =)
Gale is not that much of a Hero, but he too enjoys doing good stuff.
How are they moronic?

Both pay attention to conventions and try to fit into the male worldview of a good girl.
Its not a worldview to behave logically and normal though.
In your defense I do not know much about Shar-Clerics and from what I found out ingame, Shar is a Goddess I would never consider to worship.
Shar is a Deity of Secrets though. Which also means of manipulation.
Which means: Of decieving others.
Shadowheart could therefore play the part of a very benevolent person! Maybe even try to be heroic and cheerful, or optimistic?
You know - the clever path of being a nasty, manipulative t... otally smart, evil Cleric. =)
But she isn't.
I totally expect that at some point LarianStudio's will find a suuuuuuper believable reason for all of her character. If we recieve enough flashback material about her that is.
So that all will be forgiven and I feel so bad that I had such an unimpressed and annoyed opinion of her in the past.
I will be redeemed...
... in simping service to this high Lady (LOL)...
... and kiss her feet like the lowly male that I am. (snort)
Since this seems to be societys worldview of a good boy that is!
" She is a Wamen! All Wamen are Queeeeeeeens! "
xD