Kouen Ren (練 紅炎, Ren Kōen) is the Kou Empire's former First Imperial Prince. He was the General Commander of the Kou Empire's Western Subjugation Army and also the Governor of Balbadd when Balbadd was controlled by the Kou Empire. Kouen is the owner of three Djinns: Agares, Astaroth, and Phenex. He is a Dungeon Capturer, having successfully captured three Dungeons and is one of Judar's King Vessels. When the Kou Empire's Civil War ended, he was supposedly beheaded as the leader of the rebel army but he survived. Aladdin has faked Kouen's execution by using his magic to cast a water screen to trick everyone. This was requested by Hakuryuu, who wanted Kouen to remain living. Kouen is now living in Samon Island, which is owned by the Kou Empire, along with his brothers. 
Kouen is a tall and well-built man, with narrow red eyes and red hair that is tied back into a topknot. One of his most notable features is his goatee, which he is proud of. He has thin eyebrows and wears some eyeliner around his eyes. On his head, he wears a small Chinese headdress with a yellow gem in its center. He dons traditional clothes that are mainly red, white and black in colour along with a black flowing cape. The maroon belt he wears has a golden ornament of a beast on it. He always carries his Metal Vessels with him, which consists of a sword, a hilt ornament attached to it and a metal pauldron on his right shoulder, which resembles the head of a lion.
After the civil war, he gave Hakuryuu the limbs of his left arm and both his legs. Now, he has wooden prostheses to replace them. In Samon Island, he wears simple clothes and has his hair tied into a side ponytail on his left shoulder. He carries a walking stick to support himself. On his face, he has gotten prominent eye bags and some small wrinkles. (need refs) Although he is not very expressive, Ohtaka refers to him as "handsome".
Kouen is a calm and serious person who enjoys war. He wishes to increase both the power and territory of the Kou Empire and also, to unite the world which will be ruled by a single king. He hopes his siblings are able to aid him in this vision but if they are unwilling to, he does not wish to press them into this matter. He is shown to care a lot for his siblings and he knows them to a great measure despite the rather distant relations they have, especially towards Kougyoku. Besides this, Kouen can be sacrificial for the sake of his siblings, as shown when he surrendered to Hakuryuu during the civil war when he knew that his siblings' lives would be spared and their 'traitorous' acts pardoned. When Alibaba arrived in Samon Island and convinced Kouen and his brothers to return to the Kou Empire, Kouen suggested that Koumei should follow Alibaba back, stating that Koumei would be a better ruler in times of peace. Kouen himself also considered handing over the throne to Koumei, which Koumei isn't aware of.
In contrast to his tyrannical public image, Kouen possess an intellectual side in reading documents and learning about history. According to his brother, Kouha, Kouen is always thinking about his country but in private, he is a person greedy for knowledge. Kouen doesn't even mind hurting himself if it's for the sake of his goals. Kouen wants to solve the riddle about the Torran Language and the King Vessel and how they are linked to the past. Due to this, he is willing to side with a complete stranger, as long as he is certain that the person in question will give him the answers he seeks.
The reason why Kouen wanted to accumulate knowledge, learn the Torran language and gather information about the destroyed world, Alma Torran, was because someday he would be able to carry out his revenge for the deaths of Hakutoku, Hakuyuu and Hakuren.
Kouen is the eldest son of the first Emperor's younger brother, Koutoku Ren. Kouen and his younger brother, Koumei are full-blooded siblings, having being born from the same mother. By the age of five, Kouen has already gotten interested in history. He began to observe and study war when he was eleven. He conquered both of the 2nd and 29th Dungeons when he was only sixteen, obtaining his first and second Djinns, Agares and Astaroth. During that time, he fought together with Emperor Hakutoku and Princes Hakuyuu and Hakuren in order to unify 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 Imperial Prince. Kouen conquered the 37th Dungeon and obtained his third Djinn, Phenex when he was twenty-two years old. By then, he also started his very own war campaigns. He was present with his brother, Kouha when he 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.
Kouen returns from his campaign to attend at the funeral of the Emperor, his father. When he enters to the palace with his soldiers, everyone bows except Hakuryuu Ren. He tells his cousins, Hakuryuu and Hakuei Ren, that it's been a long time since he's seen them. Kouen pats Hakuryuu on the shoulder and praises him for having conquered a Dungeon. Koumei tells him that it would take half a day for Kouha to arrive. Kouen says that they'll just wait for him, and they'll go to his majesty's side after they are all reunited.
Some time later, he is in the palace's library reading scrolls, when Hakuryuu and Hakuei enter. Hakuei says that this appearance doesn't quite suit him, and he agrees. Kouen asks to Hakuryuu if he wants to become emperor. Hakuryuu says that that was one of his majesty's intentions, and asks Kouen why would he want to know such a thing. Kouen says that he wouldn't want to fight with a brother who shares the same blood, but there must only be one king. Hakuryuu agrees, but Kouen says he not referring to just the empire, but this world, as it only needs one king. Kouen picks up a scroll written in the Torran Language. He says that he has always wondered what exactly a King Vessel is.Kouen says that in the past, the one chosen by the Magi became a king. He adds that likewise, they're wielding a great power and shaping history, thanks to the Metal Vessels, whose creator they don't know. He says that his Djinn told him that the person who created the Metal Vessels was King Solomon, but he asks who is this King Solomon. He says that according to the Torran inscriptions, in the past, there was a certain world where people who talked in a great variety of languages lived scattered around it. He adds that because of that, different creeds and ideologies were born, countless kings rose, conflicted with each other, and soon perished.
Hakuei asks when and where did this happen, but Kouen doesn't know. He says that it appears the world in the inscriptions is completely severed from their own world, but here in the Torran Language, around the world and inside the Dungeons. Kouen asks why they only have one language. There are characters derived from the Torran Language, but since they could walk, they were born with just a single language, and again asks why. He says that because in that way, they would not perish. So that they wouldn't become extinct after conflicts are born, because they don't understand each other and become divided. He says, "So in contrast with the world of the past, our world will become one. For that reason, a single king must control the world." He says that he doesn't know if his intentions are right, but he wants to solve this riddle. He says that he wants to take them towards the heights of the "One World", and adds that for this, he wants them to lend him their strength. He calls Kougyoku Ren, his sister, as he noticed she's been there the whole time. He says that she is Vinea's master and ask if she listened well to what he said. She says that did and it was all complicated, but Kouen is an incredible person after all. She notices she's said something stupid and Kouen says that, in other words, she didn't understand. He calls Kougyoku, as she was daydreaming, and tells her that Kouha is back, but since it is already late, they will go to their father and mother's side tomorrow.
The next morning, he heads to where the funeral is with his siblings. Gyokuen Ren, his mother, greets them all. When his sisters are shocked to see what happened to their father, they all clung onto Kouen. Kouen asks Gyokuen if the Emperor died because of an illness. She confirms that and says that he was ill for a long time, but it suddenly got worse. She tells everyone to come and tell their father their farewells. She then suddenly hugs Kouen, Gyokuen says that she only has him left and asks if he, her most loved son, will support her, but Kouen pushes her back. Everyone looks at her in disgust. After this, Gyokuen is appointed the next Emperor, and that she will be assisted by the Imperial Priests in tending all national affairs, which surprises everyone. Gyokuen says that the position should have originally been inherited by Kouen, but as he is the General Commander of the Western Subjugation Army and can't be removed from that important position, he can't be next Emperor until the continental subjugation is complete. An argument ensues between everyone else and the tension starts to rise, until Hakuryuu bows to her and asks her to take the throne, and that no one but her can do it. The Imperial Priests start chanting that, which surprises Kouha, Hakuei, Kougyoku, and Seisyun Ri.
At the end of the day, he talks with his brothers about Al-Thamen. Koumei says that the "Organization" has 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 Kouha was negotiating with them, even though 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 if it is the result of their Magicians, 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.
Kouen appears in a flashback from Kouha's armies memories. When Kouha obtained his Djinn, Leraje, Kouen was with him in the Dungeon, and would have been selected by Leraje if she wouldn't have considered him a cheater for already having 3 Djinn.
Just as Alibaba Saluja is about to execute his Extreme Magic, a huge monster of Kouen's army, Shou En, eliminates the Black Djinn by vigorously breathing fire on them. Then, Kouen and three other monsters, Seishuu Ri, Kin Gaku and Kokuton Shuu appear riding a carpet. Kouha and his army are completely astonished, and claim him. Kouha is rather ashamed since has troubled a dignified person such as his brother and king. Kouen stares at the situation and, when he sees an exhausted Kouha, says to his monsters that if the enemies are too much for the others, they will deal with them with their strength.
While his Household is fighting against the Black Djinn, Kouen heads to where Kouha is injured, and heals his broken arm with the ability of one of his Djinn, Phenex. Then, he begins to give orders to vanguard. He tells them to retreat the soldiers, gather the wounded in a single place and says to Meihou Kan to emerge the vanguard with the first occupation army in the mountains under his command. Then he realises about Alibaba and Aladdin's presence, and stops with a sign the soldiers that were encircling them.
After Alibaba has introduced himself as the former third Prince of Balbadd, Kouen stares him gravely. Alibaba proclaims that has come to the battlefield to save Kouha's life, and Kouen answers that isn't Alibaba looking down on him since he has already lost his country, which embarrasses Alibaba greatly. Kouen pats his shoulder and says that he has to thank him since he did a good job in saving Kouha, and passes by from Alibaba.
When the western sky turns black, Aladdin becomes worried, and begs Kouen to retreat the soldiers or they will die, without mattering if they are from Kou or Reim, and calls Kouen old man, to everyone's astonishment. Seishuu says to Kouen don't be bothered by the nonsense spouted by this dirty little magician. When Aladdin sees that there's no reaction from Kouen's part, he says that if Kouen doesn't believe him, then the situation will be explained directly by Kouen's Djinn. Immediately, Agares, Astaroth and Phenex are summoned leaving Kouen with a surprised expression. The three Djinn say, "We are here to serve you, our "King", and the reincarnation of our past "Great King"!!!".
Kouen exclaims that the three summoned are his Djinn, and stops the scared soldiers who raised their spears towards the Djinn. After Aladdin also summons Leraje and Amon and all they ask if the Magi called them, Kouen understands that Aladdin in the fourth Magi that Judar was talking about.
He listens with a stern expression the Djinn request to their Kings: to close the Dark Spot, or otherwise this world will be annihilated. Leraje begs to the Dungeon Capturers to not let this world die or that will be the same as Alma Torran, but Astaroth cuts her off, saying that the humans of this world shouldn't even know about the existence of the other world.
This catches Kouen's attention, and when Amon refers Aladdin as Solomon's proxy he advances towards Aladdin, and questions if this other world is King Solomon's destroyed world. Then, Kouen grabs Aladdin and orders him to tell him everything right now, much to his Djinn's horror, which scares Aladdin to the point that the Magi promises Kouen tell him everything if he sends back the soldiers and helps them destroy the Medium.
Kouen agrees to that and calls Koumei Ren, Hakuryuu Ren, Hakuei Ren and Kougyoku Ren by using magic. After that, Kouen asks to the soldiers if they have finished treating the injured soldiers, and informs that as soon as the treatment for the heavily injured soldiers finishes, Kouha and him will depart. Kouha asks him if he's going to believe that absurd story, at which Kouen answers that it's not matter of believing it or not, this is part of the abyss of history he's trying to uncover, and if he would be able to solve the mystery of the world before uniting it, then from now on there won't be any need to repeat the thousands of wars that happened in the other world.
He addresses again to Aladdin and says that he will be lending him the power of all Metal Vessels users in the Kou Empire. In exchange of this, after all this through, he will tell him the truth about this world. Then, he enters in his Astaroth's Djinn Equip form and leaves with the other Dungeon Capturers lead by Aladdin.
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. Hakuei, Kougyoku, Koumei and Kouha appear with Kouen in the center and destroy them. Kouen says that he will obtain the truth about the world on the other side.
The Dungeon Conquerors are fighting against the Black Djinn but they start to become tired. Kouen keeps destroying them rather easily as, how Alibaba's points, his sword burns and blows away anything it touches. Then, a great amount of Black Djinn fusioned attack him. He spikes his sword in them, which explode and return in form of Black Rukh to the "medium".
Then, he makes his Djinn Equip disappear and changes into Agares Djinn Equip, saying "Pierce through the earth, Agares!!" With Agares' powers, Kouen digs a deep hole into the earth, which provokes an earthquake that creates a kind of a volcanic eruption. He uses the lava to absorb Magoi from the hot lava in order to use Astaroth's Metal Vessel again.
Regained the power, he starts destroying the Black Djinn without hesitation. Alibaba cannot believe that Kouen really wants to destroy ten thousands 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.
Kouen shouts towards the Black Djinn, saying that they are on his way and that whoever gets in his way will die; he inquires if any of them is going to tell him the truth, so when he kills all them he will finally obtain everything he desires. Aladdin and Alibaba are stunned after seeing that, and Kouha comments that Kouen always thinks about their country, but in private he's a person greedy for knowledge, and right now, all he's thinking about is that conversation he's gonna have with Aladdin. And adds that he's pissed off, which scares the other two.
Indeed, Kouen is angry and calls Alibaba, calling him "Brat from Balbadd", and says to get over here. He doesn't wait for Alibaba's reaction, grabs him and forces him to get into the lava flow. Hakuei makes a detailed explanation of what Kouen truly said, which is more polite and reasonable, but Aladdin has doubts about if really Kouen said it.
Kouen and Alibaba summon their Extreme Magic. Kouen's one is called Astor Inqerad, which has a dragon appearance and destroys all the Black Djinn outside of the "defensive wall" in an instant. Then, Alibaba's Extreme Magic heads against the Medium, but he alone isn't enough, so Kouen supports him and Alibaba's attack become stronger. Kouen smiles when a crack is opened in the Medium's Borg. Kouen exclaims to Alibaba that he's pathetic, and pushes him to put his back into it. Alibaba says him to shut up and activates his Extreme Magic, Amol Al-Bador Saiqa.
Alibaba's attack doesn't destroy completely the "defensive wall", and Kouen wonders how can this be. Aladdin asks Kouen why haven't his flames disappeared even though his Extreme Magic has finished. Kouen explains that Astaroth's flames last eternally unless he orders them to disappear. He adds that they can leave things as they are, as even if they don't destroy its "defensive wall" the Black Djinn that keep crawling out from it will be burned so long as they are born and eventually the "Medium" will exhaust its Magoi. Alibaba and Aladdin comment that it's a scary move. Kouen heads toward the volcano again to recover is Magoi and urges Alibaba to do the same.
Then, but, the volcano and the Black Djinn are absorbed by the Medium and a huge doll is created. Kouen wonders what the hell is that slim and huge doll and comments that the Medium became quite small; he also inquires how his flames disappeared. After Aladdin realizes about the danger, Alibaba attacks the doll, and when he's clashed against the floor Kouen begins his own attack. He doesn't have more success that Alibaba, and one of the doll's hands grabs him. He manages to cut the wrist but by then he has been stripped from his Djinn Equip and severe damaged, with his skin of the most part of his body taken off from him.
Kouen heals his injuries with Phenex, while watches how Kougyoku's attack damage the doll. Aladdin realizes which is the weak point of it, and attacks the doll with Ugo, but the doll takes the Rukh from the sand. Kouen tells him not to move recklessly, and orders Koumei to direct them. Kouha, Kougyoku and Hakuei summon their Extreme Magic and, thanks to Koumei, their attacks reach the doll.
Kouen is with Alibaba watching, and proudly says to Alibaba that their younger siblings are brave and offers Alibaba to restore his arm with Phenex's abilities. Alibaba then asks Kouen if Hakuryuu will come. Kouen answers that he doesn't know as he doesn't know whether Hakuryuu follows his orders or not. After saying that, Kouen equips Astaroth again and says that it's not time to worry about other people, they have have to move as well. All the Dungeon Capturers and Aladdin attack again, but the doll begins to steal Rukh from the sea and becomes bigger.
Kouen, Aladdin and Alibaba stare the Medium, but are unable to stop its attack. Then, a powerful lightning and Sinbad, fully Djinn Equipped, appears. When Alibaba and Aladdin exclaim his name Kouen stares him and says that Sinbad is the man who brought that army here.
When Sinbad's allies show their Metal Vessels Kouen exclaims that they are the Metal Vessel users of the Seven Seas Alliance. 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; he keeps Djinn Equipped and remains silent. He makes an exclamation, visibly surprised, when Aladdin uses Wisdom of Solomon to go inside it, when the monster begins to hesitate. After his success, he watches the Medium's destruction.
Kouen keeps his Full Djinn Equip when he and his family are surrounded by the Seven Seas Alliance. Aladdin asks them to stop, saying that Kouen promised to retreat. Kouen denies it, saying that their original objective was Magnostadt, as since the beginning he didn't want to employ Kou's Metal Vessel users only to go against Reim, and that the only thing that happened was that the opponent they had to fight changed.
After Sinbad announces the Seven Seas Alliance and Reim's newborn alliance, and declares his intentions of restoring Magnostadt, Kouen says that in this case he will be taken Aladdin with him, much to everyone's astonishment. Before Aladdin can protest, Kouen reminds him that this was what he promised, with a terrible smirk that scares Aladdin. Kouen comments that if Sinbad was just interested in gaining more territory and influence, then he would just kill them off, but it seems it's not this kind of man.
Kouen is seen in Aladdin's flashback, when he, his family and allies leave Magnostadt riding a carpet.
Alibaba arrives to Balbadd and finds Kouen sitting on a throne, wearing imperial clothes and surrounded by his Household and Kou's officials. Kou's officials try to put Alibaba in a bad situation, discussing about whether Alibaba should bow to Kouen or not. After Alibaba comes out victorious by leaving in evidence Kou's men lying, Kouen laughs, much to the officials' shock. He removes his imperial clothes and they are left to be taken by Seishuu. Kouen notices that Aladdin isn't with Alibaba and asks he what answers does he seek.
Kouen orders everybody to leave them alone and he and Alibaba end up face-to-face in a room. Neither of them says anything for a while. Kouen grew bored of the silence and demanded Alibaba to say something. Alibaba asks Kouen if he's alright talking to someone of such a low birth, which made Kouen ordering him to stop asking such a foolish questions, much to Alibaba's confusion. He explains that ten years ago he was in completely different position and it all changed due to the Emperor's death so, in the end, differences due to lineage don't exist in the world. Alibaba queries him if he's even in a position to say that sort of thing. He also mentions about the promise to make Balbadd into an autonomous republic but it's far from that in its present situation. Alibaba also wants to know why Kouen did unreasonable actions as to invade other countries. Frustrated, Kouen remarks that all Alibaba asks are questions. Then, he inquires if it ever occurred to Alibaba that there are more important things than sharing thoughts and making everyone everything. Realising that Alibaba doesn't comprehend what he said, he decides to let him talk with his younger brother, Koumei, first. Koumei enters and greets Alibaba, but immediately fells asleep standing. Kouen smacks his back harshly, demanding him to wake up before requesting Koumei to keep Alibaba company. Koumei replies that he can't be able to do so since Kouen dislocated his shoulder from his smack. Kouen tells him to stop lying and soon leaves the room.
Three years later, Kouen meets Alibaba at his current residence, Samon Island. When Alibaba asks about Kouen's missing limbs, Kouen replies he had given them to Hakuryuu. Once Alibaba remarks how old Kouen is, Kouen smacks Alibaba on the shin for being rude. While Alibaba tries to convince them to come back, Kouen suggests to Alibaba to take Koumei as Koumei is a suitable ruler in times of peace. He shares that he thought of giving the throne to Koumei once the war was over. 
Kouen is intelligent enough to know how to read and speak the Torran Language.
Kouen mastered swordsmanship when he was only nine years old and seems to be a very skilled swordsman.
Kouen mastered all of his three Djinns and is able to utilise the Djinn Equip for Agares and Astaroth. Kouen’s Djinn Equips are excellent in offensive and restoration abilities. Kouen's Metal Vessels are a sword, a hilt ornament on his sword and a spaulder. The known members of Kouen's Household are Shou En, Seishuu Ri, Kin Gaku and Kokuton Shuu.
Agares is the Gravity Djinn that Kouen obtained when he conquered the 2nd Dungeon. Agares is the Djinn of Fortitude and Creation. This Djinn dwells in the spaulder on Kouen's right arm. Kouen can manipulate Earth with this Djinn.
- Djinn Equip: In this equip, Kouen's hair becomes wild and resembles a lion, like his Djinn's and he appears childlike. He has a heavy necklace around his neck and his arms becomes scaly and turns into claws with the Djinn circle on the back of his hands. He also grows a tail. A dark tattoo around his right eye appears and he also wears a belt. He has a destructive power, as he fired a beam of light from his hands that was powerful enough to pierce through to the other side of the earth’s surface and stimulate a lava flow.
Astaroth is the Fire Djinn that Kouen obtained when he conquered the 29th Dungeon. Astaroth is the Djinn of Terror and Meditation. He dwells in Kouen's sword. This Djinn is the one Kouen uses most often.
- Djinn Weapon Equip: When his weapon is equipped whatever it touches burns and blows away. It can also create a massive blast of fire by concentrating the mass of fire at the tip of his sword.
- Djinn Equip:In this equip, Kouen's hair becomes longer and resembling snakes, like his Djinn's. He has a heavy necklace around his neck and decorations on his forearms. A dark tattoo on his stomach appears and he also wears a belt. A snake-like armour covers the surface of his body. When he equipped this form, he summons a white snake-like dragon similar with his Extreme Magic around his body. The scaly skin is hard and useful at times for defense.
|“||The Spirit of Terror and Meditation, thou who gives power to kings, bring forth the great hellfire that judges the earth!!||”|
—Kouen, Summoning Astaroth's Extreme Magic
Phenex is the Healing Djinn that Kouen obtained when he conquered the 37th Dungeon. This Djinn dwells in the hilt ornament on Kouen's sword. Phenex has the ability to heal by summoning a pink-coloured phoenix from her Metal Vessel, as shown when Kouen used her power to treat Kouha's broken arm as well as Alibaba's and his own injures. Phenex is also able to transfer limbs and organs from one person to another. Kouen used this ability to transfer his limbs to Hakuryuu for his missing legs and arm.
- The Ring of Admonition: Using Phenex's power Kouen puts inside the targets body an Egg that is warmed by targets bloodlust. In the moment enemy has concrate intention and confidence, the egg cracks to create magic ring around his head and isolate bloodlust from his body.  If the target is trying to move while the nerves are being sealed the curse may leave him into agony. 
|Quantity of Magoi||
Gyokuen is Kouen's stepmother who's standing in his way of the unification of the world and he has yet to cooperate with Gyokuen, which is unlikely. During his father's funeral, Gyokuen stepped up to him and hugged him, asking him if he will continue to support her and he is her most beloved son. Kouen merely got her off him and put her aside before proceeding to pay his last respects to his deceased father.
Hakuryuu is Kouen's younger step-brother as well as cousin who does not follow his expectations. Despite knowing that Hakuryuu also desires to obtain the Kou Empire, Kouen simply watches him and allows him to do as he please. He finds Hakuryuu’s recent rebellious attitude interesting. He also admits that he feels sorry for what their family has done to him. After the civil war and Kouen being imprisoned, he and Hakuryuu had a talk together in Kouen's cell. When Hakuryuu demanded to know why Kouen did not do anything when Gyokuen killed Hakutoku and the crown princes, Kouen replied that it was to protect the country and that he had no power to kill Gyokuen. Kouen told Hakuryuu that there were more important things than his personal revenge and that was to erase war from the world. Then, Kouen admitted that he did not want to become king, shocking Hakuryuu.
Despite having a rather tensioned relationship with Hakuryuu, Kouen cares for him and even gave him three of his limbs to replace Hakuryuu's missing ones. When Kouen learnt that Hakuryuu has 'killed' Gyokuen, he stated that he envied him because he thought he would be the one to kill her instead. Following after a bow to Hakuryuu, Kouen stated that he really regretted that he lost to Hakuryuu. After the supposedly execution of Kouen, Kouen remained alive from Hakuryuu's pardon and was sent to Samon Island to be with Koumei and Kouha.
Kouen deeply respected both Hakuyuu and Hakuren and his childhood dream was to serve under them when Hakuyuu becomes emperor. As a child when he was first introduced to them together with Koumei, he was so nervous to the point of panicking, especially when Koumei started spouting one of his lies and saying that he had some sort of chronic illness and needed to stay in his room feeding his pigeons. When he apologised profusely to Hakuyuu for Koumei's behaviour, Hakuyuu simply laughed and shrugged it off. Kouen also fought alongside with Hakuyuu and Hakuren in order to unify the Eastern Plains. When they died, he desired to get revenge but decided to hold it off for awhile due to lack of strength. (needs ref)
Hakuei is Kouen's younger step-sister and cousin, who is in cooperation with him. She admires him, and the two have a fairly good relationship despite the fact that they are step-siblings. Hakuei is one of the few people able to enter Kouen’s private space and possibly understands his quiet but occasionally blunt nature. However, even if Hakuei doesn’t hate Kouen like Hakuryuu, she has very mixed feelings since it wasn’t long ago that she was in a higher position than him until Emperor Hakutoku and the two crown princes were killed in a fire and Kouen's father, Koutoku, became the next Emperor.  Kougyoku once thought why were the both of them seemingly close and Hakuryuu is often infuriated when the two are talking to each other.
Judar gets along well with Kouen and there are numerous times when he and Kouen act together and also exchange information. Kouen's knowing of Aladdin as the fourth Magi was due to Judar. The both of them enjoy the same interest of war and when Judar told Kouen that he requested Sinbad to start a war against them, Kouen smirked and said that this was expected of the Magi of his country, showing his amusement and excitement.
Koumei is Kouen's younger and only full-blooded brother as the both of them were born from the same mother. Currently, Koumei is Kouen's advisor and although Kouen considers Koumei to be lazy, he values Koumei's opinions and abilities greatly as Koumei is actually a very intelligent person but lacks refined manners to show it. Koumei often attempts to slack off and escape from work by falling asleep while standing or making up some sort of odd lie. Kouen usually retaliates by calling him a liar or yelling at him to wake up as he hits him. He seems to be the only one who can tell when Koumei is lying.
Kouha is a general of Kouen's army, as well as one of the siblings that is the closest to Kouen. Kouha is exceptionally loyal to Kouen and often feels regret of he shows too much weaknesses or failures. He comments that both Kouen and Koumei used to take good care of him when he was younger, thus awakening his desire to take care of other people as well. Kouha originally wanted to become part of Kouen's Household rather than obtaining his own Djinn.
Kouen appears to care for Kougyoku though their interactions are limited. Out of all of his siblings and step-siblings, he seems to acknowledge her the most for her abilities as a Dungeon Conquerer. Kouen even rushed to save her and Kouha from the Medium when it attacked them. Kougyoku almost worships her older brother and wishes to be closer to him. An example of her wanting to be close to Kouen is in this scene. One night, Kougyoku expresses frustration when she sees Hakuei with Kouen, but she's relieved that it was a misunderstanding due to seeing Hakuryuu there. 
- His name in Japanese means Crimson Flame. 
- "Kouen" is the Japanese pronunciation; in Chinese his name would be Hóngyán.
- His nickname, Entei, means Flame Emperor.
- Kouen is the second person in the world to capture more than one Dungeon, right after Sinbad and before Hakuryuu Ren
- His hobby is historical studies 
- His special skill is war
- His likes spicy food and dislikes sweet food
- His favourite type of person is a smart woman and he dislikes indecisive people
- His favourite way to spend a day off is doing historical research
- He's currently worrying about how to deal with Aladdin
- His weakness is his fashion sense
- When he’s not at war, he tends to space out and think that his goatee suits him very much.
- He thinks his goatee is stylish but it's revealed to be in the top 3 most unpopular designs
- According to Morgiana, he smells like a mix between three fragrances
- Of Kouen's three Djinn, which were silhouetted in Night 87, two of them have completely different appearances to when they were introduced in Night 186
- He would have four Djinn instead of three if he was chosen by Leraje instead of Kouha
- According to Ohtaka, it's really difficult to draw Kouen with other facial expressions
- Aladdin refers to Kouen as "oji-san" (おじさん), which can be translated as uncle, mister or old man (much to the Kou Army's horror, though Kouen doesn't really mind it at all).