Duffy is a former prince who turned to a life of bloodshed after becoming the wielder of Chastice, a demon sword who feeds on blood in exchange for power. He was raised very sheltered, rarely engaging with anyone outside immediate family and never leaving the castle. It's this feeling of being a burden that made him agree to work with Chastice to escape his current situation.
While living at the castle, his outward personality is docile and recessive; he has a tendency to keep secrets and avoid people. After working with Chastice, he's more directly combative and egotistical, though who's to say if that's him or the sword?