Koumei is a thin man with long, messy red hair tied back into a ponytail and pink slanted eyes. He has acne scars on his nose and cheeks. He wears baggy robes bearing a taijitu symbol and the big dipper constellation, matching bracelets and a large earring in his left ear. He carries a fan of black feathers, which is also his Metal Vessel.
Kouha claims that Koumei is "ugly" and, in his concept art, it is stated that he doesn't have a pretty face.
Koumei constantly emits a gloomy aura. He has a serene personality that dislikes actual combat, despite being born in a military nation. It seems that Koumei, who is more of the liberal arts kind, is more cut out for shutting himself in a room, reading documents and planning military strategies, rather than engaging in actual combat. None is superior to him when it comes to military tactics .
Koumei, being the intellectual he is, immerses himself during war councils to the point he loses his sense of time, so that his sleeping and eating habits (and also his appearance) suffer as a result.
Unlike Kouen, he speaks courteously to everyone. Although outwardly Koumei appears unassuming and somewhat lazy, and shows absolutely no interest in the power struggles that exist between his siblings, he is prepared to stop at nothing to unify the world beneath the Kou Empire.
Koumei is the second son of the First Emperor's younger brother, Koutoku Ren; he and his brother Kouen have the same mother.
After the death of the Emperor and his heirs in a revolt, Koumei's father became the Second Emperor, making him the Second Prince.
Koumei conquered the 71st Dungeon and obtained his Djinn, Dantalion, when he was 20 years old.
It was Koumei who devised the system of government whereby lands conquered by the Kou Empire are forced to adhere to their customs and absorbed.
When Judar arrives in Balbadd, he greets Koumei, who says he was worried since they couldn't get in contact with him. Judar, to apologize for his lateness, gives him a gift from Sindria. Koumei is worried about the fact Judar came from such a place. Koumei, his younger brother, Kouha, and the priest go to where Kouen is. Judar says to Kouen that he has declared war to Sindria, Koumei gets angry since Judar made not only Sindria, but also the entire Seven Seas Alliance into their enemies without the Emperor permission. But Kouen starts laughing and tells everyone that for his empire that will rule the east, the day in which it will rule the world is imminent.
Koumei is in the Imperial City, with his assistant, to attend at Emperor's funeral, and welcomes Hakuei and her troops. Koumei is amazed at how quickly she arrived, Hakuei tells him that it was thanks to the help of Hakuryuu. After that, Hakuryuu Ren arrives along with his Household Vessel, and he seems a little worried with his change. They enter to the royal palace and Kouen arrives shortly after that, then Koumei bows to him.
The next day, he attends to the funeral along with his siblings. There, Gyokuen Ren greets her children. Koumei keeps a serious expression when Gyokuen grabs Kouen's arm and asks if he will support her. Kouha starts talking to Koumei and says that Gyokuen always fawned over En, and now it seems she has finally decided to move on from him. Kouha asks him if in that case they don't need to hear the testament, but Koumei only says to Kouha mind his manners and look forward, since the priests will begin to read the Emperor's testament. He is shocked and disgusted like everyone when Gyokuen is appointed as Emperor.
At the end of the day, he talks with his brothers about Al-Thamen. Koumei says that the "Organization" have changed their modus operandi as they begin to become so visible, referring to his aunt. Kouha tells Kouen that what happened in Magnostadt wasn't what they expected. When he was negotiating with them, even though Matal Mogamett faltered when Kouha mentioned Gyokuen, the chancellor refused to collaborate. Koumei mentions that Magnostadt is just like the Kou Empire was a few years ago, with their mass-production of Magic Tools, and the sudden development and increase in their military force. He suspects that Al-Thamen is behind it. Kouen states that it is the result of their Magicians that they were able to expand their strength to that extent, and that if they would come to oppose the Kou Empire, they would be a frightening country.