Actually shadowheart isn't dat good written.
She is totally cliche. Completely the "I'm mean but it's because I'm soft and broken inside please help me love me, bring me light".
TO compare, if you are looking for companions well-written, with depths, personality and consistency, you should look at pathfinder.
Amiri is a classical yet interesting barbarian with a story she didn't throw in your face.
Jaethal is a good bad companion, well-written, with spirit and a true strong-mind.
And we could keep going...
The companions ni BG3 are... nice but, from my point of view, far from amazing and memorable for some of them. There is too much inconsistency in their writing.
PS: people should be more open to the opinions different from theirs. Even if the OP can be considered clumsy in the way he present his opinion, it doesn't mean all he means is stupid.
Shadowheart is this obvious false bad girl and the game really want you to get it fast and there is no fun talking to a girl which is mean knowing she will soft with time and patience. And yet, I'm one of this gamers fond of romance.
So I would like to do this one but she is just as obvious as not-dat-good-written.