Kai is the protagonist of the story, a young soldier whose life is defined by a singular, reality-altering event. Originally a member of the Humanity Aegis Agency within the Urza Foundation, his primary duty was to stand guard over the great tombs where the four other races were sealed after humanity's victory in the Great War of the Five Races. In his original world, which enjoyed a century of peace thanks to the legendary hero known as Sid the Prophet, Kai was a dedicated and hardworking individual, known for taking his responsibilities seriously even when his colleagues did not.

The core of Kai's character is forged in the aftermath of a mysterious phenomenon called the World Reincarnation. Without warning, the entire world was overwritten in front of his eyes, transforming reality into a dark reflection where the hero Sid never existed. In this new world, humanity lost the Great War and is now subjugated by demons and other races, forcing the survivors into hiding. Crucially, while everyone else’s memories have been altered to fit the new history, Kai alone retains complete memories of the original timeline. This unique position leaves him as a stranger in a world where no one—not even his closest friends—remembers him, a concept that drives the entire narrative.

Motivated by a deep determination to reclaim the peaceful world that was stolen from him, Kai resolves to fight against the rewritten destiny. He is not a stereotypical hero chosen by prophecy but an ordinary soldier who chooses to act in the absence of the legendary Sid. His journey begins when he inherits Sid's legendary sword, Code Holder, a weapon that grants him the power to challenge the ruling races. His primary objective becomes to uncover the cause of the World Reincarnation, defeat the powerful heroes of the other races, and restore the original world for all of humanity.

Throughout his quest, Kai builds key relationships that define his path. His closest ally is Rinne, a mysterious and extraordinarily powerful girl of mixed races whom he finds chained in a tower and frees. He also reunites with Jeanne E. Anise, who was his friend and colleague in the original world; in the rewritten timeline, she is the leader of the human resistance but does not recognize him, creating a complex dynamic as Kai works to earn her trust as a stranger. His companions also include his former colleagues Aslan and Saki, whose personalities are similar to the ones he remembers, providing a faint echo of his lost life. His fights against figures like the demon hero Vanessa also reveal that the other heroes may have had a complex connection to the missing Sid, adding layers to his mission.

Kai shows significant development as the story progresses. While he initially relies heavily on the power of Sid's sword, Code Holder, he begins to grow into the role of a hero himself, learning to embody the determination and tactical thinking that defines true courage rather than solely depending on the weapon's power. His character arc moves from being a simple soldier caught in a catastrophe to an active force rebelling against fate itself, fighting not just with a sword but with the knowledge, tactics, and memories of a world that no longer exists.

In terms of abilities, Kai is a skilled swordsman, though his most notable power comes from his inherited weapon, the Code Holder. This legendary sword possesses the unique and powerful ability to cut through and purge the "correction" of reality, essentially allowing him to slice through the rewritten rules of the world itself. This power makes him a singular threat to the new world order. Complementing this is his greatest strength: the memory of the original world. He uses the knowledge of demon-fighting tactics and the history of the lost era to outmaneuver enemies who have never faced a human with such foresight, allowing him to challenge opponents who far outmatch him in raw power.