My Brosca was one of my two favorite PTs. I imagined the only reason Koshka hadn't run for the surface to escape the Carta was because of Rica, and she planned to do it as soon as Rica had a noble protector. She didn't bully for the fun of it, only when she felt she needed to make certain people remembered she was a predator and dangerous.
Ah, nice. It literally never occurred to my character to go to the surface. She didn't ask about her father to Rica, so had no idea he had escaped to the surface. (She'd always assumed she was the result of a failed attempt at noble hunting, which would explain why Kala favored Rica but hated her). Since the hole under the ground was the only world she had ever known, she assumed that's all there was. She felt she was trapped in this city with these horrible people who looked down on her and her sister, and so always acted aggressive to prove she was strong and respectable.
Imagine her shock when she mouthed off to a surface merchant, who then said words to the effect of, "I don't understand why you stay here when the doors to the surface are right there." It was like her whole world opened up. She was still toying with the idea of the surface when she met Duncan, but she still had a job to do to help Rica, and then, well... the rest is history.
She had no idea that there even were people as naive and sweet as Alistair, but when he turned out to be real, she decided it was her job to protect him from being used and toughen him up just enough that he'd survive without becoming bitter. When she found out who he was, she instantly put two and two together and realized that Eamon was going to want to put him on the throne, and not trusting Eamon, she decided the best way to make sure Alistair was protected was to put herself into position to be his mistress (and considering how Rica was treated by at least some of the nobles, there's no particular stigma to that for dwarves). By the time of the Landsmeet, she was thoroughly in love with him. And every king needs a fiercely protective assassin who's known to have a particular talent for poisons as a chancellor, right? (I really loved the idea of two kings with Brosca sisters as mistresses, btw, though apparently once Brosca becomes a paragon, Bhelen marries Rica.)
Nice. My Brosca was similarly surprised by human kindness, but with Duncan. After being treated like dirt by everyone since infancy, she became convinced that everyone (except Rica) was rotten under their shows of toughness or moral superiority. She often bullied and mouthed off to people because she felt that people show their "true colors" when insulted or challenged (especially by a "brand"). Duncan was the first person to be unfailingly polite and respectful to her even when she mouthed off to him, and embody the honor and compassion that most casted dwarves only pretend to have. He was also the first person after Rica to show he recognized her worth (which meant so much to her), stood up for her to the casted dwarves, saved her from execution, brought her to the surface, showed her a whole new world where she didn't need to be aggressive or ruthless for day-to-day living.
I imagine Brosca mellowed out once she got to the surface, since most surfacers have never heard of the caste system and don't realize she's supposed to be a "brand," so she has no reason to be rude to them. I imagine she idolized Duncan for it (much like Daveth and Alistair), and so dedicated her life to stopping the Blight and then rebuilding the Grey Warden Order in Ferelden in his memory. (What else is she gonna do? Doesn't want to return to Orzammar, but also doesn't know what to do with herself since the surface is so far and alien from everything she's ever known. At least the Wardens give her purpose and direction.)
She didn't romance anyone though. While she stopped being actively aggressive and ruthless, she also wasn't nice. She wasn't friendly, talkative, or overly interested in other people's affairs. Since DAO romances require you to be kind, polite, and supportive of your partner, I couldn't get her to kick off any romance dialogue while keeping her in-character. (Plus, she's attracted to muscle-bound dwarven men. The thicker, the better.) But that's all right. She grew a lot as a person, helped her sister find happiness, proved to herself and the world that she's more than a common thug, and found a greater purpose in life than just knocking skulls or helping her sister get knocked up. It was more than enough for her.
Did you have Morrigan along when you talked to Brosca's mother? I think it may be about the only time in the entire game when when one of your party members reacts to someone insulting you (and one of the reasons I loved Morrigan).
Actually, YES! I believe so! Doesn't she comment that you two have more in common than she thought, since Kala is just as verbally abusive to Brasca as Flemeth is to Morrigan? That was hilarious! XD