Arthur is a central figure in the Western Star, serving as its enigmatic ruler from the seat of power known as Camelot. He is a legendary king whose authority is recognized by the powerful leaders gathered around his Round Table, including figures such as Gaius Julius Caesar and Alexander the Great. Despite his status, Arthur is an intensely mysterious character whose true nature and intentions remain ambiguous for much of the story.

The most striking aspect of Arthur's appearance is that his face is perpetually obscured from view by an eerie, unnatural light. This is not a simple physical trait but a supernatural phenomenon tied to his abilities. He possesses the power to appear differently to each person who looks upon him, showing them whatever form will most effectively earn their loyalty and devotion. For example, he appears as an ambiguously brown-skinned man to Alexander and as a stunningly beautiful woman to Caesar. The intense emotional reactions these visions provoke, such as weeping tears of joy and swearing undying loyalty, suggest that this ability allows Arthur to exert a powerful, almost hypnotic influence over others. In this way, he controls his followers not merely through political power but through a more profound and unsettling manipulation of their perceptions.

Arthur's personality is complex and difficult to parse due to his enigmatic nature. He is initially presented by his followers as a wise and just king, a potential Savior King destined to bring peace and harmony to the world. However, as the narrative progresses, a more ominous picture emerges. His quest for harmony is revealed to be absolute and terrifyingly literal. He believes in a form of peace achieved through the obliteration of all individuality and conflict, a goal that he pursues without apparent malice but with chilling determination. He is not a stereotypical villain driven by greed or hatred; rather, he is an idealist whose solution to the world's chaos is its complete erasure and rebirth.

Arthur's primary motivation is the salvation of the Western Star, which is slowly dying. He becomes convinced that the only way to achieve this is to manifest the Holy Grail and bring about the "perfecta," a catastrophic event that would end the current world and grant eternal life to a select few. This goal puts him in direct opposition to Oda Nobunaga, the protagonist prophesied by Jeanne Kaguya d'Arc to be the true Savior King. Arthur sees Nobunaga as a rival destined for conflict, the adversary he must defeat to follow through on his own plan for the world. His role in the story is thus that of a major antagonist, but one whose actions are driven by a misguided and chillingly serene conviction that he is saving existence itself.

Throughout the series, Arthur's key relationships are defined by his manipulative influence. He holds absolute sway over Caesar, who becomes little more than a puppet controlled by Arthur's shadowy power. Caesar perceives Arthur as a loving light, while his own son, Cesare, sees him as an all-consuming darkness, highlighting the divergent perceptions Arthur's powers create. Leonardo da Vinci initially works for Arthur, serving as an observer of the world, but he ultimately defects to the Eastern Star with Jeanne d'Arc, seeking the true Savior King and the truth of the world apart from Arthur's vision.

In terms of development, Arthur's character shifts from a distant, revered legend to a direct, world-ending threat. His plan forces the unification of the Eastern and Western Stars against him, revealing his role as the final obstacle to peace. Notable abilities, beyond his perceptual manipulation and control over others, include his command over the Regalia, the powerful giant battle armors that define the series' combat. As the king of the Western Star, he possesses his own formidable castle-like Regalia, which serves as his stronghold and a symbol of his absolute authority. His power is so great that his defeat becomes the central challenge that Nobunaga and his allies must overcome to forge a new world.