Atsuya, also known formally as Setsuyuu, is a character from the kingdom of En who serves as the primary antagonist in the story arc concerning the province of Gen. His official title is the self-proclaimed regent of Gen, a position he assumed after determining that his father, the rightful governor, was unfit to rule the province. Atsuya is a human male who, on the surface, presents himself as a kind and exceptionally capable administrator, appearing to be a man who has genuinely improved the welfare and prosperity of the people under his care. He is described as being highly qualified and seemingly a more fitting leader than the king of En himself, Shouryuu, whose unconventional and seemingly flaky behavior leads many, including Atsuya, to underestimate his shrewdness.

However, this image of a benevolent and competent leader masks a deeply flawed and arrogant personality. Atsuya is driven by an enormous ego and a complex of superiority that leads him to genuinely believe he would be a better king than the divinely chosen ruler. His primary motivation is not the welfare of Gen, but his own pride and an inability to withstand any form of humiliation or personal failure. This is most clearly demonstrated by his decision to rebel against the king by trying to become an over-king, a position that would allow him to rule in all but name. He takes this path because he fears the shame he would suffer if he were to climb Mount Hou and be rejected by the kirin, the sacred beast who chooses the king, proving he was never meant to rule.

Atsuya's role in the story is as a rebel leader who instigates a conflict early in King Shouryuu's reign. He kidnaps the kirin of En, Rokuta, and imprisons the king's consul to Gen, using them as leverage to demand power from the throne. He justifies his rebellion by claiming that the king's incompetence has led to the decline of Gen, and he incites the people against the crown by focusing on a critical flooding problem that the king appears to be neglecting. As the story progresses, his true nature is exposed. When the king shrewdly resolves the flooding issue by ordering the construction of levees, Atsuya loses the support of his own people, who rebel against his army when he attempts to destroy the dam.

His key relationships are primarily defined by manipulation and control. He employs a young man named Kouya, a boy raised by a demon who is searching for a place to belong, as a devoted servant. Kouya serves Atsuya faithfully, finding in him the acceptance he has always wanted. Atsuya also has a deeply abusive relationship with his own father, the former governor of Gen. To maintain the illusion of his father's illness and his own legitimate rise to power, Atsuya imprisoned his father in a cell and forced a different man with his tongue cut out to act as an impostor governor in the inner palace.

Atsuya's character development, or rather the revelation of his true self, is a process of unmasking. Initially perceived as a promising and capable leader, he is gradually shown to be a fanatical and delusional tyrant who cannot admit error. When his schemes begin to fail and his crimes are exposed by Rokuta, he refuses to take responsibility, instead blaming his advisors for the entire rebellion. In a final confrontation with King Shouryuu, who has disguised himself as a guard, Atsuya is given a choice to surrender or face the king in a one-on-one sword battle for the throne. He initially acquiesces to the king's demands for surrender, but his twisted pride prevents him from accepting defeat gracefully. He attempts to attack the king from behind, leading to his own death. The king's shirei, a familiar spirit, tears through Atsuya's neck, and the king then beheads him to end his suffering. In terms of notable abilities, Atsuya is depicted as a capable swordsman, enough to engage in a duel with the king, though his final, treacherous attack shows that his martial skill is ultimately overshadowed by his corrupt character.