Atsuyu (斡由), whose formal given name is Setsuyuu (接祐), is the self-proclaimed regent of the province of Gen in the kingdom of En. Early in Shoryu's reign, Atsuyu attempts a rebellion against the king of En, claiming that Gen's prosperity has declined due to the incompetence of the new king.
However, his actions are actually due to his massive ego and superiority complex, in which he has deluded himself into thinking he would be a better king. After years of manipulation and abuse of everyone around him, particularly of Kouya, he is killed while cheating in a duel against Shoryu.
During the end of Kyou-ou's reign, Atsuyu deemed his father, the governor of Gen, to be unfit to rule the province properly. He became the regent of Gen supposedly because his father had become ill and had chosen to retire in the Inner Palace of the governor's palace, leaving all matters of governing to his son. Atsuyu also ends up employing Kouya, a young boy who was raised by a demon, as a devoted servant. Atsuyu becomes known in Gen for his apparent kindness and capable administration of the province to ensuring the welfare of the people of Gen.
However, when he perceives that the king of En is incompetent and neglecting Gen, Atsuyu incites his people against the king and begins to amass an army against Shoryu. His plans involve kidnapping Enki and imprisoning the king's consul to Gen, Ribi. In exchange, Atsuyu demands to be made the over-king, allowing Shoryu to continue the way he has while Atsuyu governs En. In response, Shoryu orders the kingsmen to build levees for Gen, resolving the primary concern of the people of Gen, and demands that Atsuyu return Enki.
As Atsuyu begins to lose support for his rebellion, he becomes increasingly fanatical about pressing forward with his plans to usurp the king of En. His true character becomes clear once Enki learns what has truly happened to the governor of Gen: Atsuyu had imprisoned his own father within the deepest confines of the palace while a chained up man in the Inner Palace with his tongue cut out has been forced to be an impostor for Atsuyu's purposes of maintaining the illusion of his father being ill. Atsuyu's father describes Atsuyu has being highly capable in many areas, but is so arrogant he believes himself to be incapable of error, always blaming his mistakes on others in order to maintain his own self-image of perfection. His attempt to be over-king reflects his inability to withstand humiliation; instead of climbing Mount Hou and seeking out Enki, he feared the humiliation he would suffer if he were to learn that he was not meant to be the king of En, and thus seeks to rule En without being king.
Soon after, Enki exposes Atsuyu's true nature to the court at Gen by revealing the actual fate of the governor of Gen and the impostor governor. Atsuyu, still unwilling to admit his mistake, blames the entire rebellion on his advisers and begs Enki to allow him to personally explain to the king the circumstances in Gen. When Enki refuses these terms, Atsuyu lashes out at his court until Shoryu, having disguised himself as a guard named Fuukan, stops him at sword point. Though Atsuyu acquiesces to Shoryu's immediate demands to surrender and offers of leniency, Atsuyu attempts to attack the king when his back is turned. To protect the king, Enki orders his shirei Rikaku to attack; Rikaku tears through Atsuyu's neck. To spare Atsuyu from any more suffering, Shoryu beheads him.