Merlin himself is the most important character. He is the one with the magic and knowledge, and the story likely revolves around his adventures.
King Arthur is probably an important character too. He is often associated with Merlin in many legends, and Merlin is usually his advisor. Their relationship would be a significant part of the story.
Morgana could be an important character as well. In some Merlin - related stories, she is a complex figure, sometimes an adversary, with her own powers and motives that can add depth and conflict to the plot.