I recently finished reading a PDF version of 'Dewa Arthur' and the cast felt quite sprawling at first. The central trio is Arthur, of course, the boy destined to become the Dewa, his loyal and more strategically-minded friend Gawain, and the enigmatic magical girl Guinevere who has her own ties to the ancient conflict. They're the emotional core. The antagonists are where it gets interesting though—there's Mordred, who isn't just a straightforward villain but Arthur's half-brother, and their rivalry is layered with family drama and conflicting ideas of justice. Then you have Morgana, a sorceress whose motivations shift from pure malice to something more tragic as her backstory with Arthur's father is revealed. The story also spends a good amount of time on side characters like Lancelot, a knight from a rival kingdom whose honor code constantly clashes with his growing respect for Arthur, and Merlin, who's less of a wise old man and more of a mischievous, almost untrustworthy guide pulling strings from the shadows. What I liked is that even the 'villains' have understandable reasons for what they do, which makes the political and magical conflicts feel weightier.
Honestly, I sometimes forgot about the king, Uther Pendragon, because he's more of a background figure whose legacy haunts everyone rather than an active player. The key dynamics really revolve around Arthur's generation trying to fix or overthrow the world the previous one broke.