Vivienne is probably one of the best characters in DAI, if only because of the number of people who react negatively to her.
Mary Kirby must be satisfied...
As Sera eloquently says:
Actually, if I'm not mistaken, Sera's exact words were "She's a b!tch. And she knows it. She better."
That one line immediately made me doubly appreciate Vivienne, and triply appreciate Sera at the same time. Sera's bluntness and her effective reflection on Vivienne's character make them so interesting.
I honestly wasn't sold on Sera at the time. She was funny, but her face was ugly on first close-up, because I prefer characters to have realistic faces that aren't too realistic. Plus BioWare didn't fix her god-awful fringe like I'd hoped. Fortunately, banter with Dorian explains why her fringe is so bad.
That said, Sera was always going to be one of the 3 characters I knew I'd romance.