Colin Morgan as Merlin
A powerful wizard still learning to use his gifts in a land where magic is forbidden on pain of death, Merlin has learned his destiny is to protect Arthur and help him become a great and just king who brings peace and prosperity to Camelot. In the meantime he must work as Arthur's manservant, enduring the menial work as he slowly learns that the prince isn't really the entitled prat he seems. Fortunately Merlin has the guidance of Gaius, who knows Merlin's secret and once knew something of magic himself, and the friendhip of fellow servant Gwen, who serves the Lady Morgana. Above all he must hide his abilities from the wrath of Uther, even as he uses them to shield Arthur from the sorcery that still holds sway in Camelot.
Bradley James as Prince Arthur
Blessed with good looks and a superb knack for all kinds of combat honed by years of training, the young crown prince is his father's only son. As heir to Uther's throne he feels the future of Camelot on his shoulders, and is constantly pressing himself to live up to his father's expectations of him. Yet he feels a knight's duty to the people he protects most of all, and is not afraid to stand up to the king for what is right. He also knows he must someday have a queen, and is troubled by the feelings he has for the beautiful, plain-spoken servant girl, Gwen.
Richard Wilson as Gaius
Gaius, the court physician, is a trusted counselor of the king and a master of lore both natural and magical. Though Uther does not know that Gaius harbors a young magician, he trusts Gaius's judgment on threats of a supernatural nature. Gaius knows that his knowledge and steady guidance will help the sometimes reckless Merlin to stay alive long enough to fulfill his destiny, and comes quickly to feel a paternal affection for this son he never had.
Katie McGrath as Lady Morgana
Angel Coulby as Guinevere
John Hurt as the Voice of the Great Dragon
Santiago Cabrera as Lancelot
Anthony Head as King Uther Pendragon
Uther is driven by the need to protect Camelot from all threats, and there is no threat greater than magic. To Uther, magic and all its practitioners are evil, bar none, and for the past two decades as levied a death penalty on anyone found guilty of working it. He justifies his ruthlessness by the strength and peace he has brought Camelot, and though not incapable of admitting a mistake will not be gainsayed in his stamping out of sorcery. Behind this lies guilt and anger over a long-buried, secret bargain with servants of the Old Magic. After years of childlessness he was gifted with a son, but he was horrified to learn that the price would be the life of his beloved wife. Only Gaius knows Uther's dark secret.