Just pulled the book back off the shelf to check, because I always mix her up with that other faerie, Valerian. Nicasia's the daughter of the Queen of the Undersea, so she's a mermaid-sidhe princess, basically swimming in privilege. She's introduced as part of Cardan's nasty little clique, the one who seems to genuinely loathe Jude from jump street, maybe because Jude's a mortal in their space, maybe because Cardan's weirdly obsessed with her. Her role really crystallizes in the later books, though. She's a political player in her own right, not just arm candy. Her mom's power means she's a piece on the board, and she learns to move herself. The betrayal with the crown... that was cold. She's less of a one-note bully and more of a calculating noble who starts off as a mean girl and ends up a dangerous ally-by-necessity. I found her arc way more interesting on a re-read, when you're not just seeing her through Jude's furious, humiliated eyes.
That scene on the beach in 'The Wicked King' where they have to bargain? Total shift. You see the steel under the scales. She's not asking for help; she's negotiating from a position of hidden strength. Her role is to show that in Elfhame, everyone is using everyone, and the personal is always, always political. Also, she gives us that iconic line about Cardan's bed, which is just perfectly petty and glorious.