Atsuyu

Atsuyu, whose formal given name is Setsuyuu, 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.

[Spoiler] 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. [/Spoiler]

(Source: The Twelve Kingdoms Wiki)