Ei Sei grew up as a political hostage in the State of Zhao. He was abandoned by his father, Sou Jou, when the latter successfully fled Zhao. He was also forsaken by everyone (including his own mother because she blames him for all her misfortune). She even tried to kill him on several occasions. He was forced to fend for himself and forced to steal food in order to eat. The citizens of Zhao hate Ei Sei because of his connection to Qin and the throne. Zhao has a deep hatred towards Qin. Nobody is allowed to kill Ei Sei, however they do torture him and nobody is willing to come to his aid. Due to Ei Sei's father being the king of Qin, a rescue plan was set in motion to save Ei Sei and bring him back to Qin. This was done with the help of black market merchants. They initially refused, but their leader Shi Ka however after her talk with Ei Sei accepted the job. In order to survive the harsh treatment in Kantan, Ei Sei had to learn to fight at which he is very capable at. However he is alone against an entire city of people against him and is still a child, therefore easily overpowered. When people of Qin told Ei Sei that they want to bring him to Qin, he panicked and collapsed. Ei Sei is haunted by inner demons due to harsh treatment and only managed to break free of that hatred thanks to Shi Ka. Because She freed him from that darkness, he can feel pain again which he blocked in order to protect himself. Ultimately He managed to get back to Qin, but not before the black merchants sacrificed their lives in order to save Ei Sei.
Ei Sei's life is in danger from his half brother Sei Kyou and the minister Ketsu Shi who want both his death and the throne. Helped by those loyal to him, Ei Sei was able to escape from the revolt in the Royal Palace, but he is not out of danger yet. Hyou tells him to wait for Shin in case if he dies. After they met and dealt with the assassin, they then try to escape the army with the help Ka Ryo Ten and make their way to meet Shou Bun Kun. Sei Kyou is very cold towards Shin but that's just temporary. After Shin defeated Muta, and joined Shou Bun Kun, they make their way to Yo Tan Wa the king of the mountain tribe to ask his help against Sei Kyou. Ei Sei persuades the Queen, with his idea to unite all of the middle kingdom. This succeeds due to the fact that she wanted united the mountain tribes much like Ei Sei wants to unite the plains. After that they move to the capital of Qin and he fights with the main force while Shin and the others are using a secret passage to the throne room. When the battle is won, Sei Kyou attacks Ei Sei accusing him to be a lowly commoner, and after that he is beaten half dead by Ei Sei, he even persuades Ou Ki to retreat. Once he reclaimed his throne he revived the negotiations wit the mountain people once created by Boku Kou.
Ei Sei who is still a king without any authority talks to Shou Bun Kun about Wei. Wei blocks for Qin the path to the east. They talk about how formidable Wei is. Somewhere before the campaign, Ei Sei instead of killing the traitors who were part of the rebellion, he preserved them into countering Ryo Fui ever expanding influence. He ordered everyone to do as if the rebellion never happened. He gave the authority over Ketsu Shi's faction over to Shi Shi. Ei Sei keeps himself informed about the campaign, but isn't allowed to interfere. At the end of the campaign, he was in Anken while watching Shin and Ka Ryo Ten.
In order to ensure the royal bloodline, someone from the Royal Harem named Kou comes to Ei Sei's chambers. However nothing happened, they didn't speak a word at all. Ei Sei is reading scrolls every night, but still calls for her every night. The fifth night was different as Kou actively asked questions to the king. It appears that the king summoned her to get some reading time as his schedule is quite busy. Ei Sei trusted Kou enough to tell about his past when he was held captive in Zhao’s Kantan.
The influential people loyal to the Ei Sei faction are being killed and there are assassins headed towards the Royal Palace. Their aim is to kill the king. Luckily Shin is there to guard the king. Ei Sei is in his chambers with Kou. As usual Ei Sei is reading while Kou is watching the sky. However to Shin surprise one of the assassins he knows from the war. Suddenly Ei Sei hears a noise as he then sees Shin. The king quickly realises that there must be assassins aiming for his head as this would be the only reason for Shin to be here. Ei Sei knows an escape route and are all now headed towards it. Meanwhile Shin talks to Ei Sei about the last war adn his involvement in it. However the escape route is blocked and the king knows only one person who could have known about this passage. This being Ryo Fui. Not long after they are surrounded by assassins. Ei Sei seems however to recognise their clans. Shin is forced an assassin he knows as Kyou Kai. Shin has improved leaps and bounds, but is no match against Kyou Kai. The other assassins however believe Kyou Kai to have fallen and are heading for the king themselves. Shin and kyou Kai call a truce and work together to fight the other clans. Shin and Kyou Kai are being overpowered by the enemy's sheer numbers. Luckily Shou Bun Kun, Heki and Shi Shi managed to get there in time to aid them. Kyou Kai passed out as well as Shin. The king ordered Shin to protect Kyou Kai. However the others want to imprison and interrogate her. Shin refuses as then Kyou Kai escapes. Ei Sei who knows that Ryo Fui was behind this and know has proof of this thanks to the assassins they captured states that the war between him and Ryo Fui has begun.
Ei Sei knows that Ryo Fui was the one who was behind the assassination attempt, but can't lift a finger to punish him. Even worse Ryo Fui admitted it and still the only thing Ei Sei could do is to pass it as a joke. Ei Sei is being introduced to Ryo Fui Faction and his four pillars. One of the four pillars Mou Bu asks for a royal hearing. He want's to revive the six general system. However none of the Ei Sei faction is pleased with that most of all not the king. Even in the Ryo Fui Faction they aren't pleased with that idea. Ei Sei is angered and starts hitting his throne as Shin then stops him. Ei Sei talks to Shin about the difference in power and how Ryo Fui came to power. Moreover they talk about the future and what steps to take.
Ei Sei is still a king without power and didn't have any say in sending an army to invade Han. However when Zhao invaded them, he quickly takes the helm and ordered to emergency conscription. The problem rose who was going to lead them. Both Shou Bun Kun and Shou Hei Kun called for Ou Ki who now entered the throne room with next to him Shin much to the kings surprise. Even more when Ou Ki asked everyone to vacate the throne room as he wanted to talk to Ei Sei alone. They have a talk about Ei Sei’s past. Ou Ki then remembers that he has a message for the king from king Sho. With this Ei Sei appoints Ou Ki to Commander-in-Chief and Ou Ki leaves for the battlefield. Ei Sei and Shin have a small opportunity to talk as well. Ei Sei tells that he is actually surprised that Shin would seek out training from Ou Ki. He also warns him not to underestimate Zhao.
While discussing how Ryo's faction has grown, Sei's faction begun to wonder how they could defeat them with their tiny number of followers. Mister Shi Shi revealed the existence of a third faction, that even Ryo Fui had trouble dealing with. He informed them that an envoy from that faction appeared at his estate the day before, and gave him a letter with the royal seal, which was hidden during Sei Kyou Rebellion by Sei on the eve of his escape. Shou Bun Kun told them that the third faction is the Royal Harem and that it is ruled by Sei's mother.
Heki thought that, as Sei's mother she would definitely accept Sei's request for an alliance, but Sei explained that the relationship between him and his mother is not that simple. Shou Bun Kun told them the reason they abstained from making contact with the harem up until then was because they did not want the harem faction to get close to Sei, because of how women of high authority in the past have led to the downfall of their respective dynasties.
Sei had Shi Shi open the seal to take a look, but he was surprised to see that the page was blank. This meant either that the letter was a prank, or that the Queen Mother had decided to ally herself with either their faction or Ryo Fui.
Yet somehow, he also has a feeling that it holds a completely different meaning. But more importantly the matter of the seal on the letter, means they have purposely stamped it with the royal seal and delivered it into their hands, the meaning there was extremely obvious. Sei says they should know that they would not sit by and overlook the brazen act and yet, they still chose to send it. In other words, whoever sent the letter wants them to turn their gaze towards the Royal Harem. Shi Shi tells them that the Queen has three ladies in waiting under her, managing the harem. And behind them are the three great official families Han, Kai, and Ryou, and even since olden times they've been a minor power backing the harem. And if they were join forces with them, their faction would be nearly even with chancellor Ryo's camp.
Were they to ally with Ryo Fui, they would become completely powerless to resist them. So no matter which path they chose they couldn't avoid the harem faction. Shou Bun Kun then explain's the Queen Mother is poison, and the three families are her poisonous fangs. Looking at it simply, it might appear that their faction would swell in numbers, but the question was whether or not they would truly be able to cooperate together, united against Ryo Fui. If anything the harem could drag them into their own ridiculous schemes could make the alliance the more terrifying choice, because they would be unable to repel Ryo Fui's forces. The next day, Sei went to the harem. As Sei stands clear across the room his mother has him come closer to see his face.
See remarks on how he has become quite a fine man in the small amount of time since they last met. Sei says its the same for he, and her beauty has yet to fade. He then tells her to stop with the innocent act, and asks her to hand over the copy of the royal seal. He tells he for a prank, it was going too far, and were the matter to had been exposed, there would be an uproar over her being tried for the crime of betraying their state. She tells him traitor of the state does sound severe, but its funny for those words to come up between them. Considering the lifestyle they lead in Zhao. She reminds him of the abuse they suffered, and with not a single source of money, they were forced to sully themselves just to be able to eat.
But now he is the almighty King of the great state of Qin and she is its Queen Mother, and how on paper they posses the two highest positions in the kingdom. And there should not be a single person who stands above the two of them. She then explains to Chou Kou the reason her and Sei had avoided each other was because they didn't want to remind each other of those hellish days in Zhao. When Sei asks he why she copied the seal and sent the letter, she replies by saying that she just felt like seeing his face again after such a long time.
Sei then turns to take his leave but she asks him if their was a request he wanted to make. He tells her he's changed his mind. She then gets him to request for the assistance of the Harem Faction. She then tells him she will send her reply in a few days. A few days later, a messenger arrived at one of Sei's estates. The answer given was "yes." A few days later, You comes to Sei at night to request aid for Kou. Sei tells her that Kou would speak of her every now and then, and she is just as she described, inclined to take action in unusual ways.
Sei has a messenger go to wake up his personal doctors to heal Kou. After she is stabilized, she informs Sei that the harm joining his camp was a lie. And that Ryo Fui and his mother were betrothed before his birth. And that the previous night they had an affair. He then has Kou get some rest. Later, he puts the pieces together. So the strange antics his mother committed were all for the purpose of baiting out Ryo Fui, and the objective of meeting him was to reaffirm her apathy towards him.
The next day he calls Shou Bun Kun and Shi Shi together. He tells them Ryo Fui has revealed an opening, and it's time for them to switch to the offensive. And they will strike directly at the center of his power. He has them leak the information of Ryo Fui and the Queen Mother's affair. And when the heads of the three great families switch to Ryo Fui's camp, this was seen as an unnatural move for the Queen Mother's Faction to side with the enemy of her own son. Which lead lead people to believe the rumor. Sei then takes the advantage to increase his numbers and disrupt Ryo Fui's camp.
Upon hearing the news on the Coalition Army invasion, Sei gathered his top officials and strategist and discussed about the way they should set the defensive forces in order to save their Qin from total annihilation. After all suggestion were heard, they decided to bet all their chances on a united stand on the walls of Kankoku Pass.
During the last days of the Battle of Kankoku Pass, general Ri Boku of Zhao secretly gathered an army of more than 30,000 strong aiming to take the southern pass of Bu and reach the Qin capital, Kanyou. As a last attempt to save his kingdom, king Ei Sei took his personal guard and decided to make his last stand in the city of Sai which lies just before the path leading to Kanyou. After consulting Shou Hei Kun and taking his approval, Sei took his personal army and left in secret for Sai. There, he met Shin and his army, who managed to escape their pursuers and reach Sai as well. Sei gathered the people of Sai and gave a moving speech managing to raise their morale and convince them to take up arms and assist him in his last stand to save his kingdom.
After boosting the citizens' morale, Sei, Ten and Shin set the plans for the city's defense strategy but Ten noted that there was a lack of commanders to organize all four posts on the walls. The headquarters were placed near the frontal gate, from where Ei Sei would be able to tour the four walls and raise the morale when needed. Ten and Mou Ki, who also arrived earlier, were stationed at the headquarters to act as the head strategists.
As the days went by, the situation was getting unbearable for the Sai defenders. Sei noticed that and on the night of the second day he decided to tour the camps and talk with his people. Speaking to them as an everyday man rather than a king, Sei managed to reach into their hearts and give them the courage to continue fighting this battle for survival with an even higher spirit.
On the fifth day, the Qin soldiers were truly at their limit. Many of them started falling on their knees, even dropping dead, before even being attacked by their enemies. Sei saw that and made a desperate and extremely dangerous move, despite his advisers' objections; he entered the battle himself on the southern wall, yelling at his men not to give up just yet.
Witnessing a bunch of kids fighting in front of him, Sei sent his personal garrison to save them. An enemy captain, Sou, noticed a kid with a shiny armor leading the militia figured out that he must be someone important and moved to take him out. Sei moved quick and cut the attacker's arm but another soldier that followed managed to surprise the king and wound him in the neck area. Just as the Zhao soldier was about to finish the helpless Sei off, Shin arrived and killed him. In a frenzy, Shin massacred all the Zhao soldiers around Sei and secured his life. When the night fell, the militia started to lose faith due to the king being heavily wounded. Shū Bun Kun suggested that the king should be evacuated, but Sei refused.
On the morning of the sixth day, the loss of their cornerstone, Sei, had turned the militia into empty husks. On the other hand, the Ri Boku Army were more than ever determined to end this war. Ri Boku ordered his men to go on an all out assault on all four sides of the city and placed reserve units to prevent the Qin king from escaping. Their main goal was to capture Sei, dead or alive. Seeing his men on the verge of despair, Sei gathered all his strength and managed to stand up on his horse and tour the walls once again. When the soldiers saw their king being alive they were once again filled with courage and the will to fight. Despite the pressure, Sai managed to last until the night fell. Sei and Ten went to Shin's post on the wall to help wake him up and treat his wounds, and, along with the other members of the unit, they chatted and slept through the night.
On the 7th day the signs for Sai were ominous, as the militia couldn't hold any longer and the city was finally breached. However it was then, when help finally arrived, the Yo Tan Wa Army. Sei revealed that he had secretly contacted his old ally before leaving Kanyou, but this move was only known to Shou Bun Kun, because Sei didn't want to take the risk of a leak or giving false hope, in case the call wasn't answered.
The mountain warriors were too much for the already worn out Zhao army to handle and they were forced to retreat. After the battle's end, Sei expressed his gratitude to Yo Tan Wa and invited to stay for a while, but she refused, saying that she was to return to her own campaign immediately. So they postponed their talk for another time, when they would both have the time to spare.
The same night, while everyone was resting, Sei was restless. Shin went to have a chat with him, and it was then, when Sei revealed the guilt he felt for manipulating the people of Sai into fighting and sacrificing half their numbers. Shin comforted his friend by saying that the people were smart enough to know what they were getting into. Instead, it was the inspiring king's speech that moved them and led them to put their lives on the line. Such a powerful and reliable leader had Sei become.
In the morning, everything was wrapped up and Sei led his army back to Kanyou, where he reunited with Kou.
With the first of Sei's offspring (and its mother) posed as no more than a side note, Kou stirred the hearts of Sei's faction when she gave birth to his second child, a little girl named Rei.