Like any good princess, Millerna has been neatly bartered off in some unspecified arrangement that results in a betrothal between the son of King Aston's Chief Advisor and herself. Normally in such stories, the fiancee turns out to be some terrible fellow with warts, bad breath and no sense of romance.
However, Millerna's husband-to-be, Dryden Fassa, is handsome, intelligent, well-educated and -travelled, funny, proud, a bit exuberant, slightly excitable, kind, sensitive and the inheritor to probably the greatest fortune in Asturia; not to mention the fortune he has amassed with his own talent, which is implied to be even greater than his father's.
He becomes involved in the adventure midway, but his knowledge, excellent manners, connections and resources make him a welcome addition to the team, and a worthy rival to Allen in Millerna's heart. Unfortunately he proves woefully inadequate to the extraordinary situations that come up once they're all back in Asturia and he loses some faith in himself. They make a good peacetime couple, but of course, the series is set in wartime, when dashing warriors outshine cunning merchants in the eyes of impressionable young girls.