Ei Sei is the thirty-first king of the State of Qin, having ascended to the throne at the young age of thirteen. His origins are marked by profound hardship, as he was born in the Zhao capital of Kantan while his family was held in political exile. His father, the king of Qin, abandoned him and his mother to escape back to their homeland, leaving the young prince to endure a brutal childhood. During this time, he was subjected to relentless physical and emotional abuse by the citizens of Zhao, who despised him for his Qin lineage, and was also neglected and targeted by his own mother, who blamed him for her misfortunes. This traumatic upbringing had a deep psychological impact, causing Ei Sei to become emotionally detached, distrustful of others, and physically numb to pain. He was haunted by inner demons, which he perceived as the ghosts of the horrific massacre at Chouhei, and was forced to fend for himself in a hostile environment.

A turning point in his early life came when a group of black-market merchants from Qin, led by a woman named Shi Ka, attempted to rescue him. Shi Ka was the first person with whom Ei Sei felt safe, and through her kindness and sacrifice, she managed to break down the emotional barriers he had constructed. Shi Ka gave her life to ensure his escape back to Qin, an act for which Ei Sei remained forever grateful, and it was through her that he regained his ability to feel and began to see goodness in the world.

As a person, Ei Sei initially presents a levelheaded but cold and pragmatic demeanor. He displayed a ruthless side early on, executing a wounded enemy without hesitation when a more merciful path was suggested. However, this exterior belies a deep capacity for care and loyalty towards those who serve him faithfully. He is driven by an immense and unwavering resolve, which is matched by a profound charisma that inspires fierce devotion in his followers. His defining motivation, and the central goal of his life, is the unification of all of China under the banner of Qin. He envisions putting an end to the endless wars of the Warring States period, and to achieve this, he plans to dissolve all existing state borders, including Qin's own. In their place, he intends to establish a new legalist system where all citizens have the same rights and duties, where there is no privilege or discrimination, and where the law stands above every person to prevent future conflicts. He is fully aware that this ambition will require an immense amount of bloodshed, but he believes it is a necessary price for lasting peace.

In the story, Ei Sei begins as a king with little real power, his authority largely a formality while powerful ministers like the merchant Ryo Fui control the court. His early role involves fighting to reclaim his throne from a rebellion led by his own half-brother, Sei Kyou, an event that forces him into an alliance with the young war orphan Shin. This partnership becomes the cornerstone of his reign. Over time, Ei Sei engages in a silent but intense political war with Ryo Fui, slowly building a faction of loyal retainers and navigating complex plots to consolidate his power until his coming-of-age ceremony grants him full authority. His leadership is most famously demonstrated during the defense of Sai, where he donned golden armor and stood on the front lines, delivering a powerful speech that rallied the terrified and untrained citizens to fight against a massive invading army.

His key relationships are central to his development. His bond with Shin begins with Ei Sei viewing him merely as a tool, a sword to be used. However, through shared ordeals and mutual ambition, Shin becomes his most trusted friend, his rival, and the man he calls his sword for the unification of China. He shares a deep and trusting relationship with his concubine, Kou, who is a beloved companion and the mother of his daughter, Rei. He confided his traumatic past in her, demonstrating a rare moment of vulnerability. His half-brother, Sei Kyou, is a figure of hatred and pity who initially tries to kill him, but eventually develops a grudging respect for Ei Sei's vision. His mother, the Queen Mother, is another source of deep personal conflict, as she has repeatedly tried to have him killed, yet he still seeks to stop and heal her rather than destroy her. His relationship with the powerful mountain tribe leader, Yo Tan Wa, evolves from hostility to a strong alliance built on their coinciding dreams of unification. His primary political rival, Ryo Fui, is a figure he hates but respects as a formidable opponent.

Ei Sei undergoes significant development, transforming from cold, traumatized child into a just and inspiring leader. His experiences with Shi Ka, Shin, and his own people help him regain his humanity and capacity for trust. He evolves from a powerless figurehead into a king who is the true heart of his faction, capable of making difficult political decisions and leading his people from the front in their darkest hour.

Notable abilities include exceptional leadership and charisma, which allow him to motivate and inspire everyone from weary soldiers to terrified civilians. He is also politically astute and highly intelligent, able to outmaneuver his rivals and control the flow of information to his advantage. While not a trained soldier, Ei Sei possesses considerable physical capabilities and is proficient with a sword, able to fight effectively when necessary.