Kouen is a tall, well-built man with red eyes and red hair tied back into a little ponytail, a goatee, thin eyebrows, and a small black hat with a yellow gem in the center. He wears traditional clothes that signify his royal status, and a black cape. He carries his Metal Vessels with him: a sword, a hilt ornament on it and a metal pauldron on his right shoulder, which resembles a lion's head.
Though he is not very expressive, Ohtaka refers to him as "handsome".
Kouen seems to be a calm and serious person, but he also likes wars and wants to increase the power and territory of the Kou Empire. Kouen has a desire to become the only king in the world. He wants his siblings to help him but doesn't wish to force them if they aren't willing to.
In contrast to his tyrannical public image, Kouen also possess an intellectual side in reading documents and learning about history.According to his brother Kouha, even if he always thinks about the country, in private he's a person greedy for knowledge. He doesn't mind hurting himself if it's for the sake of his goals.
He also wants to solve the riddle of the Torran Language and the King Vessel and how they are linked to the past. To do so, he is willing to side even with a complete stranger, for as long as he is certain that the person in question will give him the answers he seeks.
Kouen is the eldest son of the first Emperor's younger brother, Koutoku Ren; he and his brother Koumei have the same mother. He became interested in history and war at an early age, often studying history scrolls and observing wars.
Kouen conquered both the 2nd and 29th Dungeons when he was 16, obtaining his first and second Djinns, Agares and Astaroth. At that time, he fought with Emperor Hakutoku together with Hakuyuu and Hakuren in unifying the Eastern Plains,
After the death of the first Emperor and his heirs in a fire, Kouen's father became the second Emperor, making him the first Prince.
When he was 22, he conquered the 37th Dungeon and obtained his third Djinn, Phenex, and started his own war campaign.
He was present when his brother Kouha obtained his Djinn, Leraje.
Kouen is in the Kou Empire's Army's Base in Balbadd's territory, with the Left General, Kokuhyou Shuu, and the Right General, Seiryuu Ri, waiting for Judar's return. When Judar goes to talk with him, Kouen says that, for Judar to make him wait he must've brought him an interesting present. Judar says he has, that he told Sinbad, "Why don't you fight a war, against us?" Kouha and Koumei are surprised that he did this without the Emperor's permission. Kouen starts laughing, and says that's very befitting for the Magi of his empire. Kouen tells everyone that for his empire that will rule the east, the day in which it will rule the world is imminent.