Kai is the childhood friend of Zard, a hero who vanquished the evil dragon Diabolos at seven years old, bringing peace to the world. When Diabolos resurrected four years later, Zard embarked on a second battle and vanished, fueling Kai's quest to investigate Zard's belief in human-demon coexistence and uncover his fate.

In 4996, Kai begins his journey to explore the possibility of peace. He encounters Izayoi, a boy who communicates with demons through his heart, and Gend, a brown-skinned swordfighter and sole survivor of the extinct Dark Elves. Recognizing their relevance to Zard's vision, Kai forms a traveling party with them.

As a former knight, Kai draws on combat expertise and leadership. His personal story arc within the manga delves into past connections from his knighthood, forming part of the larger "Tenbin no Dairi" (Agent of Balance) narrative that extends beyond the original series.

Kai evolves from personal inquiry to active advocacy for coexistence. Witnessing Izayoi’s bond with demons and Gend’s vengeance-driven quest against Diabolos—who annihilated his people—Kai weaves their struggles into his purpose. He becomes a unifying force, channeling Gend’s pursuit of justice and Izayoi’s empathy toward realizing Zard’s ideal.

The "Tenbin no Dairi" arc further chronicles his commitment to balance, depicting his ongoing mediation in human-demon conflicts and cementing his role as a steadfast agent of Zard’s philosophy beyond the initial journey.