Arthur is the charismatic and enigmatic leader of the World Council's special agency, the Knights of the Round, and serves as the representative of the Normal Realm. His past is marked by abandonment; after living in the Normal Realm as a child, he was cast out to the Heavenly Realm. This early experience of displacement and coldness culminated in a fateful encounter on a snowy night with the one who would become his dearest friend, Santa Claus, who gave him the will to live. This friendship from his youth forms a core part of his history, and he has vowed to protect the vast world that brought his friend joy.

In the present, Arthur operates in a moral gray zone. While he publicly aligns himself with the World Council's official policy of non-interference with the Divine Gate, he privately believes that the current separation of the unified worlds is unnatural and harmful. Driven by this conviction, he secretly seeks to use the Gate's power to restore the original connections between the realms, a goal that requires calculated and purposeful action, even if it means defying the very institution he represents. He is a strategist who balances political maneuvering with covert operations, embodying a rare blend of statesman and frontline commander. His guiding principle is to restore the world to its original state, and he pursues this goal with unwavering determination, having assembled a group of Adapters, including the protagonists, to aid him.

Arthur's role in the story is pivotal as the mastermind who sets the main plot into motion. He is the one who gathers the Adapters and officially announces the quest for the Divine Gate, moving the pieces into place for the conflict to come. His primary weapon is the renowned bayonet-type Driver known as Excalibur, which he wields with the Light attribute, fighting in a style that is both noble and efficient. His authority is symbolized by his floating fortress, the utopia of Avalon, where he waits with a faint, knowing smile.

Key relationships further define his character. His bond with his old friend, Santa Claus, is significant; Santa ultimately entrusts his mission to Arthur, leaving the ideal realm of Avalon to him. Arthur also has a notable dynamic with the trickster god Loki, whose unpredictable nature and unclear motives stand in opposition to Arthur's plans. In one of the anime's final acts, Arthur explicitly states that his wish is to destroy the Divine Gate in order to return the world to its original, pre-chaos state. As his development progresses from the leader known as Arthur to the self-proclaimed Saint King Arthur, he takes on the duties of the Sacred Calendar King. His ultimate transformation is into Saint God Arthur, a being who has chosen the path of a god, discarding his kingdom and people as no longer necessary, demonstrating the extreme lengths to which his convictions can drive him.