Duncan is from Highever, but little is known of his history. He is soft spoken (almost monotone) and has a calm demeanor. He is humble and thoughtful in his actions. Alistair speaks very highly of him, as do others whom the Warden meets in his adventures. However, he is almost ruthless in doing what is necessary to stop the Blight, as that is his main objective.
Duncan is a dual wielding warrior of unknown level and ability. His fighting skill is hinted to be quite impressive as he does not fear travelling alone in Darkspawn infested areas - indeed, he does tell others he is in no danger in such areas. And during the Battle of Ostagar he single handedly defeats an Ogre with his dual weapons.
However when the King was killed by an Ogre in the battle of Ferelden, this calm demeanor he holds vanishes and attacks with rage, which, just like the king, leads him to his death soon after.