Colin Morgan as Merlin: The series protagonist, a young, remarkably talented sorcerer who attempts to keep his powers secret and will one day become the greatest wizard in the world. He is also Arthur's friend, despite the initial animosity between the two.
Bradley James as Prince Arthur: the headstrong, arrogant and reckless but kind and benevolent son of King Uther, and future King of Camelot. He is friends with Merlin, and the pair have saved one another's lives on numerous occasions.
Richard Wilson as Gaius: Camelot's court physician and one of the few who know Merlin's secret. Gaius used to be a sorcerer himself, and is something of an alchemist and magical scholar. He has a dry sense of humour and sees Merlin as the son he never had.
Anthony Head as Uther Pendragon: Arthur's father and current king of Camelot. Uther is depicted as a ruthless and sometimes cruel man, but he cares deeply about his subjects and friends. However, he is frequently blinded by his hatred for all forms of magic.
Angel Coulby as Guinevere (Gwen): Morgana's humble maid servant, and her friend. She has an open, friendly personality but is occasionally antagonistic with Arthur.
Katie McGrath as Morgana: King Uther's ward and the daughter of his late best friend. Morgana is best friends with Gwen despite their class differences and is also secretly a "seer" - a person who has prophetic powers.
John Hurt as the voice of the The Great Dragon: Merlin often visits the Dragon when in a predicament, because the Dragon knows what Merlin is destined for. The Dragon is also one of the few who know his secret. The Dragon appears omniscient at times, and his motivations are unknown beyond his desire to be released from the dungeons of Camelot.
Michelle Ryan as Nimueh: The main antagonist of the first series, Nimueh is here portrayed as a beautiful, powerful, but amoral sorceress, bent on overthrowing Uther and Arthur and restoring magic to Camelot.
