Rosalind is from a relatively small noble family, who are closely related to the De Courci's. She was elevated to the position of De Courci High Captain after her predecessor was murdered by one of Belor's assasins (and her main rival for the position was disgraced). Outwardly she seems to play the role of the 'party girl' but this is entirely a ruse, as she in fact is a competent strategist. There are rumours that she isn't willing to shy away from more underhanded tactics to protect the city.
Although she is married to a De Courci scion, the High Captain might in fact be gay. After the 'For a Muse of Fire' freeform she started having a relationship with Ambriel Chermes (although this was relatively brief, since Ambriel died in the war on Belor and Selena). She may, on the other, hand have just been trying to gain information from the Port.