Casstus is a High-Elf Bard, and member of the noble House Wolfe. Born as a noble and middle son (two older brothers and one younger?), he was a recipient of a card from the Deck of Many Things.
After the mutiny at sea, Casstus sneakily convinced the crew to rename the ship “The Sea Wolf” and to fly a flag very similar to that of his house. Casstus’ motivation seems to be to advance the goals of House Wolfe (and thereby High King Tenzu IV) by bringing order to chaos, crushing the rebellion in Panthia, and becoming a conquest paladin.