Koumei Ren (練 紅明, Ren Kōmei) is the Kou Empire's former Second Imperial Prince. He was a General of the Kou Empire's Western Subjugation Army along with his brothers, Kouen and Kouha. Koumei is the owner of the Djinn, Dantalion. He is a Dungeon Capturer, capturing one Dungeon and is one of Judar's King Vessels. After the Kou Empire's Civil War, he was sentenced to be exiled by Hakuryuu upon Kouen's request for mercy. He is now currently living in Samon Island, which is owned by the Kou Empire, along with his brothers but was then taken back to Rakushou by Alibaba to help the Kou Empire regain its stability.
Koumei is a thin man with very long, messy red hair tied in a ponytail and slanted pink 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. His clothes are frequently in some form of disarray. He carries a fan of black feathers, which is also his Metal Vessel.
His concept art deems him "plain" and less attractive than his brothers.
After the three year timeskip Koumei has cut his hair, now sporting a short and shaggy style similar to Hakuren's.  He also now has a large scar on his chest due to the injury he received from Takeruhiko Yamato during the Kou Empire Civil War. 
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.
After the death of the Emperor and his heirs in a revolt, Koumei's father became the Second Emperor, making him the Second Prince.
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.
Koumei is working in the palace when his brother, Kouen Ren, calls him and the other Kou Metal Vessel users through long-range Clairvoyance Magic to go to Magnostadt. He answers "Ok, ok, I heard you" and enters in his Dantalion Djinn Equip form.
Aladdin is in trouble because he doesn't have any staff and cannot use his Magic decently when Hakuei uses Paimon's power to catch the Black Djinn and throws them to where the Kou warriors are. Koumei, Hakuei, Kougyoku Ren and Kouha appear with Kouen in the center and destroy them. Then, Aladdin calls Hakuei, who is surprised to see him and asks what is he doing there. Koumei asks Hakuei is she knows Aladdin, and how are they related. Hakuei, smiling, answers that Aladdin saved her life.
Koumei is fighting against the Black Djinn along with the other Dungeon Capturers. He uses Dantalion's powers, Dante Al-Thais, which consists in attacking drawing the stars. His attacks slice the Black Djinn. Koumei comments that he's so tired, as his Djinn isn't that fit for battles. Alibaba Saluja is surprised when he sees his abilities, and wonders what the hell are those strange attacks. A few after, he and the majority of the other Dungeon Capturers are tired. He watches Kouen destroying several Black Djinn, and when his brother goes out from his Astaroth Djinn Equip and Alibaba asks if he has run out of Magoi, Koumei answers that Kouen is just switching his Equip.
After regaining his power, Kouen starts destroying the Black Djinn without hesitation. Alibaba cannot believe that Kouen really wants to destroy ten thousand Black Djinn. Aladdin exclaims that doing that is too reckless, and that they have to stop him or his body will be severely damaged. Koumei replies that it's useless, as Kouen wouldn't listen to them, and adds that Kouen is the kind of person that doesn't mind hurting himself. Alibaba, with a face of comprehension and respect, begins to say that Kouen is doing this for the sake of the world. Koumei interrupts him, saying that he didn't mean that, which surprises Alibaba, who is not understanding. Koumei comments that Kouen has an excited face. When Kouen and Alibaba starts summoning their Extreme Magic at the same time, Koumei says that they have to protect them so no Black Djinn gets close to them, at which Hakuei agrees.
After Alibaba summons his Extreme Magic, Koumei asks Kouen how long should he keep transferring the potential damages away from the city, and adds that it's quite exhausting. Kouha is behind him cheering him on. He's very surprised when Kougyoku's attacks damage the doll.
Kouen orders Koumei to direct the attacks of Kouha, Kougyoku, Hakuei and Aladdin to avoid the doll of absorbing them. Thanks to Dantalion's abilities, the attacks reach and damage the doll. Then, the Dungeon Capturers attack again, with Kouen and Alibaba joining the fight. Koumei directs the attacks, but after the impact they realize that the doll begins to steal Rukh from the sea and becomes bigger.
The doll damages Kougyoku and Kouha, and Koumei uses his power to teleport an entire mountain to damage the doll -much for the priest's surprise- but the attack fails, as the doll remains unharmed.
Alibaba exclaims that they have to use their Djinn Equip to help the Seven Seas Alliance, but Koumei objects that their Magoi is already at its limit. He fully recovers his Magoi thanks to Scheherazade and summons his Extreme Magic along with all the Dungeon Capturers gathered in the battlefield. He then watches the Medium falling to pieces, hurt and exhausted. His Djinn Equip wears off and he falls on his knees.
Koumei and his family are surrounded by the Seven Seas Alliance. When Darius Leoxses suggests that they may be hiding a trump card, Koumei answers that they don't know if the Seven Seas Alliance is hiding a trump card somewhere either, and raises his fan.
Dantalion is the Space-Time Djinn that Koumei obtained when he conquered the 71st Dungeon. His Metal Vessel is a fan with a golden handle and black feathers. Koumei can't do anything without his Metal Vessel.
- Djinn Equip: In his Djinn Equip form, Koumei has horns, which are of an appearance reminiscent of Greek mythology, on each side of his head and his hair loose. The djinn equip that wraps Koumei’s body with hard armour is simple and emits a calmer air about it, compared to those of his siblings. He has the eight pointed star symbol on the back of each hand and a mantle coiled around his back with constellations that imitates the night sky drawn on it. Anything that befalls the boundaries created by the light from his hands can be teleported to other places. Despite the simplicity, Koumei shoulders a very important role.
- Dante Al-Thais (Big Dipper Transfer Circle) : This space-governing technique is a magic most ideal for intellectual battles. By drawing the constellation, Big Dipper, with his fingers it acts as a magic circle that can teleport objects or persons through it to another constellation. He is also able to transfer the potential damages that a certain area may receive away from the place and can even teleport a great amount of material like a mountain, but it is quite exhausting for him.
Kouen is Koumei's older brother. He trusts Koumei to the point he is willing to entrust important situations to him. Koumei has decided to follow Kouen to the last. He shows great respect for Kouen, calling him "My Brother and King", and he also knows very well how greedy Kouen is when it comes to knowledge.
Kouha is Koumei's simple-minded and mischievous younger half-brother. Though Kouha teases him about his appearance, Koumei is a caring and responsible older brother. Koumei seems to worry about the well-being of Kouha, even defending him in battles, as when they fought against the Medium together. Koumei sometimes saves Kouha from the naive things he does. There are also many occasions where they have worked together and, despite having different birth mothers, their relationship is pretty good.
Despite Judar being the Kou Empire’s high priest and reliable backbone, Koumei has little patience for his eccentric antics, chastising him for disappearing without notice and playing pranks on the imperial family. Judar tends to breed worry on Koumei about his non-standard actions.
Hakuei is Koumei's beautiful and valiant younger step-sister. To Koumei, she, who fights as a military tactician, is one of his comrades. Previously, their positions were reversed and it seems that Koumei is still unaccustomed to the change. Koumei displays concern for her safety, as when they fought against the Medium together. Koumei also calls her 'Lady Hakuei'.
When one of his subordinates notes how she is not going to take part in the fight to regain the capital from her little brother, stating that her actions would demoralize her army, Koumei responds by telling him never to utter those words again, admitting: "Our emotions are getting in our way, just like hers".
However, after finding out that Hakuei betrayed the Kou Empire by forming an alliance with the Seven Seas, Koumei seemed very shocked and terrified at the princess' decision.
- His name means Crimson Light or Bright Crimson.
- "Koumei" is the Japanese pronunciation; in Chinese his name would be Hóngmíng.
- He might be based on the famous Chinese strategist Kongming (also "Koumei" in Japanese), with whom he shares several traits.
- His hobby is feeding pigeons.
- His weakness is his bad skin, which is caused by lack of sleep.
- His special skill is military strategy.
- His type of woman is a good housewife and he doesn't like loud and lively people.
- He's currently worried about the decrease in his physical strength.
- His favorite food is smoked squid and he dislikes meat.
- His favorite way to spend a day off is staying indoors.
- He doesn't believe in fortune telling.
- "Pimples" and "Shaggy" are some of the nicknames given to Koumei by Judar.
- Kouha also seems to like teasing him about his looks, claiming that Koumei is "ugly" on one occasion and then calling him handsome on another.
- Koumei has a habit of rubbing the back of his head with one hand when embarrassed or frustrated.