Kai Renald Roulle is a former soldier of the Uralis Kingdom who becomes a key ally to Fushi during the second major arc of the story. He is a man in his thirties with the weathered and capable appearance of a seasoned fighter, marked distinctly by a noticeable scar on his face. Kai is recruited by the prince Bonchien for a crucial mission to defend the city of Renril from an impending attack by the monstrous Nokkers, eventually becoming one of three humans chosen to join Fushi's Immortal Army.

Kai's background is defined by hardship and a complex family history. Growing up, he lived with his father and sick grandmother. His childhood was marked by physical abuse from his father, a period of his life where he felt a deep, passive despair and even wished for an end to his suffering. When Nokkers attacked and destroyed his village, he and his entire family unexpectedly survived. This event, followed by Prince Bonchien's invitation for survivors to move to the capital, became a turning point. Kai chose to leave his past behind and run away to join the Uralis army. Over time, his perspective matured. As his father lay on his own deathbed due to illness, Kai was able to let go of the resentment from his childhood, finding a sense of closure and peace. With his father and grandmother eventually passing, Kai became a man unburdened by family ties and determined to live a life of purpose for as long as he could.

In terms of personality, Kai presents as grounded, observant, and patient. He is a steady presence who demonstrates considerable emotional intelligence. For instance, he is able to perceive Fushi's restlessness and impatience when dealing with the political protocols of Renril's royal court. To alleviate this tension and build a connection, Kai invites Fushi to play a strategic board game similar to Reversi. During their game, he shares his painful history not for sympathy, but to illustrate his philosophy on life and to explain his profound motivation for joining Fushi's cause. He views Fushi's ability to create immortal vessels not with fear or revulsion, but as an attractive and honorable opportunity. He willingly offers his own body to become one of Fushi's forms, believing that in doing so, he could continue to serve and live a life of purpose forever.

Kai's relationships are central to his role. His primary alliance is with Prince Bonchien, a man he clearly respects and who trusts him in return. It is Bon who identifies Kai, along with Hairo Rich and Messer Robin Bastar, as the ideal candidates to become Fushi's immortal companions, valuing their loyalty, resilience, and individual strengths. With Fushi, Kai develops a unique bond. He is one of the first humans to willingly and enthusiastically offer himself to Fushi for the purpose of becoming a vessel, a gesture that stands in stark contrast to Fushi's own initial discomfort with using human forms. His calm demeanor and willingness to share his past help to bridge the gap between Fushi and his new human allies.

Throughout the story, Kai proves his worth as an expert combatant, fighting alongside Fushi and the others against the Nokker horde. He willingly sacrifices his own life during the intense battle to defend Renril, falling in combat. However, his story does not end there. Years later, after the conflict is over, Fushi chooses to resurrect Kai and his fallen comrades. Having been given a second chance at life, Kai ultimately chose to live as an ordinary man, free from the cycle of endless battle. He later passed away again, not from violence, but from a drug overdose, a quiet and human end to a life marked by both violence and a search for meaning.