Arthur Pendragon (played by Bradley James) was the prince of Camelot, heir to the throne and will apparently be one day be "The Once and Future King who will unite the Land of Albion." Arthur is 21 years old, and in the first series he officially becomes Crown Prince of Camelot at his coming of age ceremony. By series four he becomes King of Camelot due to the death of his father.
Arthur is the son of King Uther Pendragon and has the same domineering attitude as his father. He is initially described as a bully by Merlin and Gwen, and beats Merlin twice in battles in The Dragon's Call. Merlin and Arthur both have an instant dislike to one another, although Arthur notes that there is something about Merlin that he "just can't put his finger on" and believes he is special in some way. When Merlin saves his life, Merlin is rewarded with a job in the Royal household as Arthur's manservant. Eventually they become friends. Their friendship is characterized by banter and insults, with them often playing tricks on each other, but there is a bond. This is more obvious when they are alone as Arthur becomes friendlier and more open, although this never occurs around others. Arthur undertakes a life-endangering quest to find the antidote to the magical poison that is killing Merlin, but he is unaware that Merlin is a wizard and has saved his life numerous times using magic.
In "Excalibur" it was revealed that Arthur was conceived using magic as his mother Igraine was barren. She died during his birth, leading to his father's hatred of all magic and especially the witch Nimueh, who cast the spell. Few know this (Uther, Gaius and Nimueh) and all have sworn to secrecy or have no way of telling him. It is not known if Arthur takes his father's stance on magic and reports any use of sorcery in Camelot, although he carries out his father's orders. He believes himself to be a "big disappointment" to his father due to his father's attitude towards him, something Uther proclaims as his own fault, not Arthur's.
Arthur is a master swordsman, being the reigning Champion of Camelot and Leader of the Knights of Camelot. He is also the only one who can wield Excalibur, due to his destiny as King Arthur (however Uther did wield the sword on one occasion). He is rather proud of his physical prowess, so much so he swears Merlin and Gaius to secrecy when he is made to believe that Merlin knocked him out, and he strongly denies that Morgana beat him in sword fights when they were children. In series 2, when he believes the Knights are letting him win due to his position as Prince, he disguises himself as a commoner to ensure a fair fight at a jousting tournament and prove his skills once and for all (Although only himself, Gwen and Merlin are aware that he entered the tournament at all, Arthur realising that he needed to be more humble and allowing the peasant he had recruited to serve as his 'public' face in the tournament to take the credit).
Although he is shown to be rather arrogant, stubborn when he doesn't get his way and becoming particularly hostile when his honour or skills are questioned, Arthur has put his own standing and life in jeopardy for what he believes is right, such as releasing Lancelot from prison after producing false documents of nobility as he recognised Lancelot's noble nature despite his lack of noble birth, and on another occasion drinking what he believed to be poison to save Merlin and break a curse he had unintentionally unleashed upon the kingdom. At the end of the series, Arthur gets bitten by the questing beast to save Merlin. After Merlin saves Arthur by bartering his own life away, Arthur and Uther talk, and Arthur confesses he feels there's someone watching over him, though he doesn't realise it to be Merlin.