I mean, her name is Shadowheart for starters lol.
It's not like she named herself like this. It's a nickname given to her by the Church of Sharr who is literally a goddess of shadows.
She acts like a teenager who's convinced she's NOT going through a phase. She has tsundere goth emo girl written all over her.
And your opinion is based on what interactions with her? Even before patch 5 Shadowheart was extremely nice and amicable as long as you weren't creepy or reckless.
