He is an expert swordsman and a good leader of his troops. In fact many of the men in the castle follow him rather than Ferris. He is loyal to his Uncle and helps him until he learns of his treachery, then conspires with Halt, Will, and Horace to stop the Outsiders. He does not really want to be king, but he feels it is his duty to his country; Halt believes he will make a good king.
He decides to be a king after Halt calls a meeting of all the people who could become king if Sean refuses the crown and asks his nephew if he would like to see any of these people as King of Clonmel. Sean realizes he wouldn't; if they took the throne Clonmel would be in trouble. So he becomes the King and a treaty is arranged between Clonmel and Araluen, making the two countries friends.
Sean is a young man with dark curly hair and a ready, if tired, smile. Despite his relative youth, he is a great swordsman and leader, faithful to his country and wants what is best for it. This gives him a good foundation to be a king.