|“||I only believe in lust for power. The disgraceful greed of all men. I do as I want using that.||”|
|“||Shin of the Hi Shin Unit, though you may serve Sei as his sword, do not forget that you are also his shield||”|
— Last words to Shin
He is dressed in clothing befitting of royalty. In the later chapters, he has grown significantly to the point of being taller than his half-brother Ei Sei.
In the first several arcs, the young prince was extremely cruel and ruthless in completing his goals. He thought that people who were not royal had lower blood and were thus worthless; therefore, he was extremely angry that Ei Sei became king when his mother did not have any blood connection to the throne.
He used to make people do his bidding because he thought a king was allowed to do whatever he wanted and saw that people actually did what he wanted. For example, he made his followers form a bridge by standing in the water and bowing forward, despite there being a bridge less than twenty metres away.
After his release from incarceration, his character seemed to stay the same. However, before Sei rode out towards Sai, he entrusted Kanyou to Seikyou. This level of trust, something he had not ever received in his life before shocked him.
Towards the end of his life and after learning of Sei's ultimate goal, he had resolved to become Sei's shield, protecting him from the level of bloodshed that would undoubtedly occur on Sei's path to uniting China. It would be a bloodbath so severe that it would make Chouhei look like a mere drop in a bucket. He also concluded that if Sei were to falter, then he would take his place and finish the job. Sei's ambition and trust in him made him want to help and understand Sei. He had also developed feelings for Rui, and eventually was noble and brave enough to try and save her in Tonryuu, despite his various injuries. Unfortunately, he was unable to help Sei due to his death, and asked Shin to act in his stead, and protect Sei from the brutality of the world.
In the first episode, Sei Kyou appears in a scene where he is surrounded by dead people while he is holding a sword. We don't know if he killed them himself or ordered someone to do it. Somewhere before the start of the series, he made Rankai his slave by torturing him.
Sei Kyou has teamed up with Ketsu Shi, a powerful minister at the Royal Palace to take the throne from his half brother. In fact, it seems that Sei Kyou's goal was to see his brother dead rather than get control of the throne. Ei Sei managed to escape assassination, however, and together with powerful allies, like the mountain tribes, Shou Bun Kun and Shin, he suppressed his brother's rebellion. Sei Kyou was imprisoned in a palace for three years after his defeat.
When approached by his half brother, he agrees to assist him in defeating Ryo Fui, in exchange for his freedom and all his retainers who have been confined with him. He tells Ei Sei that the thought of a mongrel like him on the throne makes him sick to the stomach but the thought of a filthy commoner like Ryo Fui in power of the court is even harder to swallow.
After that, Ei Sei's faction began to rapidly grow because the men who were released were distant relatives of the royal family. Every single one of them were powerful men who hold great influence in their own right. According to Shou Bun Kun, Sei Kyou is gaining power too. Ei Sei informed everyone that they had ten days to scout as many people as possible so they could appoint Shou Bun Kun as Chancellor of the Left.
As the Ri Boku Army was approaching Kanyou, Ei Sei chose to leave the capital and set a defence point at the city of Sai. While the king was defending Sai, Seikyou was appointed as a substitute, much to his surprise. As Sei entrusted Kanyou to him, he learnt of Sei's ambition to rule all of China.
In the time between Sai's defence, Seikyou had changed dramatically. His leadership abilities had grown, and his followers were obeying him with more zeal than ever. Ryofui noticed the rise of this new enemy, and quickly hatched a plot to rid himself of Seikyou. He orchestrated an army to attack Tonryuu, the city where Rui was staying, to goad Seikyou into leading an army there.
Once Seikyou arrived and seemingly routed the enemy forces, he was ensnared in a trap by Ho Kaku, one of his general was killed by the other, and he was imprisoned. Meanwhile, rumours that he had betrayed Sei had been spread, severely weakening his faction's influence.
As the Hi Shin Unit was descending upon the city, he convinced the guards to release him and take him to Rui. Along the way, he was accosted by guards, and defeated them at the cost of Juhaku's life and a severe wound to himself.
He freed Rui and started escaping with her, but had to stay behind in order to hold off the guards with Shukan, a guard who had helped him escape, and whose family (alongside those who had aided them earlier) would get wealth and safety in return for Shukan's aid.
After being goaded by Ho Kaku into recklessly charging in by mentioning Rui, he was fatally wounded but was able to kill Ho Kaku. He died in Rui's arms after lamenting his inability to be at Sei's side to witness him acheiving his goal and then telling Shin to protect Sei along his path, and to make sure he didn't waver along the way.
In the first few arcs, Seikyou had little martial abilities, relying on his status and Rankai for protection. After his release from imprisonment, his leadership skills grew, and he was able to convince the guards in Tonryuu to switch sides and join him by pointing out how they could be erased as witnesses by his enemies. At the same time, he was willing to reward them and their families in exchange for their loyalty as he needed witnesses to show that he was innocent. He also showed proficiency with a sword, being able to hold his own against several guards and defeat them, although he was wounded and needed some protection from Ju Haku. Prior to his death, his charisma and leadership skills had grown high enough that once he died, 90 percent of his faction remained loyal rather than defecting compared to the predicted 50 percent.
- Sei Kyou hated Ei Sei for taking the throne away, but part of his desire to see the latter killed might have been due to the fact that his mother fell sick and died not soon after Ei Sei's mother became the king's consort.