Jukai is a character whose life is defined by a profound transformation from a perpetrator of violence into a healer, and he serves as the adoptive father and mentor to the protagonist, Hyakkimaru. Before becoming a doctor, Jukai served as a torturer for a warlord named Lord Shiba, where he was responsible for the brutal crucifixion and killing of many people. This violent past haunted him deeply. The turning point occurred when his own wife was captured and he was ordered to crucify her, an act that caused him to break down and attempt suicide by throwing himself off a cliff. He survived, rescued by a foreign ship, and it was during his time abroad that he learned advanced techniques for making prosthetics.

Upon returning, Jukai dedicated his life to atoning for his past sins by becoming a doctor who offered his services and prosthetics for free to those in need. He is a kind and selfless individual, yet he is perpetually burdened by guilt over the atrocities he committed. He is also a workaholic, often tirelessly creating limbs for the living and even for the corpses found on battlefields, as a way to restore a sense of humanity and dignity to the war-torn dead. His unassuming and shabby appearance reflects his ascetic life, as he charges his patients little to nothing for his life-changing prosthetics.

Jukai’s primary role in the story is as the guardian and trainer of Hyakkimaru. He discovered the infant Hyakkimaru as a newborn floating in a river, missing most of his limbs, facial features, and even internal organs. Moved by the baby’s sheer will to live, Jukai decided to raise him, naming him Hyakkimaru and crafting a full set of prosthetic limbs for him. As Hyakkimaru grew, Jukai taught him swordsmanship and how to defend himself, a decision born from necessity as the boy’s unique nature began to attract demons. When Hyakkimaru killed a demon and regained one of his real legs, Jukai realized that his son’s lost body parts had been sacrificed to demons. Understanding that Hyakkimaru would need to hunt these creatures to become whole, Jukai prepared him for his journey, installing hidden swords within his prosthetic arms.

His key relationships are central to his character arc. His bond with Hyakkimaru is that of a devoted and compassionate father, providing the unconditional love and care that the boy never received from his biological parents. Before Hyakkimaru, Jukai had taken in another orphan, Kaname, who became his student. In a tragic echo of his past, Kaname discovered that Jukai was responsible for the death of his parents and left him, discarding the prosthetic leg Jukai had made for him. This abandonment deepened Jukai’s sense of isolation and unworthiness. Later in the story, the pure and trusting relationship with Hyakkimaru offers Jukai a form of redemption that his previous attempts at atonement could not provide.

Throughout the narrative, Jukai develops from a broken man seeking death to one who finds purpose in caring for others. His final decision to remain in a collapsing castle, meeting his end alongside other characters, can be seen as his acceptance of fate and a final act of atonement, rather than a continuation of his earlier suicidal despair.

Jukai possesses notable abilities that are both medical and practical. He is a genius craftsman, capable of creating highly realistic and functional prosthetic limbs, hands, and eyes from materials like wood and ceramic, which Hyakkimaru can control with impressive dexterity. In addition to his skills as a doctor, he is also a capable combat instructor, having taught Hyakkimaru the sword-fighting techniques necessary to survive against the demons he would face.