Josak Gurrier, alias Grillet Josak, is a half-human, half-Mazoku spy and soldier serving Shin Makoku. Born in Big Shimaron to a human mother and Mazoku father, he faced isolation in an impoverished settlement for those associated with demons. Abandoned early by his mother at a church or temple, his slow aging—a mixed-heritage trait—led to confinement in a harsh, guarded village with scarce resources. He endured until age twelve, when Dunheely Weller and his son Conrad rescued him and other half-bloods during a night operation, relocating them to Shin Makoku's Luttenberg. There, Josak received formal education and military training.

He presents a cheerful, happy-go-lucky demeanor, frequently employing humor and maintaining a smile even under pressure. Fiercely loyal to Shin Makoku, he prioritizes his country's well-being and acts fearlessly in critical moments. This patriotism, forged by his traumatic childhood, fuels his declaration never to betray his homeland despite his mixed blood. Josak voices opinions openly, even to stern superiors like Gwendal von Voltaire, yet follows orders promptly. Pragmatic and adaptable, he utilizes cross-dressing as an espionage survival tactic, carrying a pink dress as a cherished good-luck charm crediting it with saving his life. He enjoys grilled lamb meat, admires Anissina, and harbors deep affection for Conrad, loved since childhood. He also expresses a wish that Queen Celi had been his mother.

His relationship with Conrad Weller is rooted in shared history. Growing up together in Luttenberg, bonded by their half-blood status and Dunheely's influence, both enlisted in Shin Makoku's military within the specialized Luttenberg platoon for half-bloods. They became the sole survivors of the Battle of Arnold, earning Conrad the epithet "Lion of Lütenberg." Josak initially idolized Conrad as a hero, but their dynamic strained over Conrad's rejection of military glory and political ambitions. Josak later shifted his admiration to Gwendal von Voltaire, valuing his fair leadership and integrity. Serving as Gwendal's primary intelligence operative, Josak conducts espionage, disguises, and covert operations, benefiting from his superior's nonjudgmental support for his cross-dressing.

Regarding Yuuri Shibuya, Josak initially viewed the young king with skepticism due to his inexperience. Witnessing Yuuri's resolve during missions—especially in perilous territories like Seisakoku—cultivated deep loyalty, leading Josak to address him as "young master" and serve as a reliable bodyguard. He also interacts with Ken Murata, offering philosophical guidance such as advising him to maintain hope.

Trained at Shin Makoku's Naval Academy, Josak is a skilled warrior proficient in swordsmanship and combat. His espionage expertise includes mastery of disguises, multilingualism, and communication via carrier pigeons. He adapts to environments restrictive to full-blooded Mazoku, such as souseki-filled areas, and is immune to racial land effects. On missions, he aids Yuuri's group with reconnaissance and survival tactics like navigating human territories or identifying edible plants. A minor subplot involves him receiving Morgif's jewel from Yuuri, its ultimate fate unspecified.

Throughout the series, Josak evolves from a soldier fixated on past military ideals into a more nuanced protector. He reconciles Conrad's choices, supports Yuuri's vision for human-demon peace, and cements his role as a trusted agent under Gwendal. His backstory includes unresolved elements like an unknown birth date, a consequence of his mother's neglect and societal discrimination against mixed-blood children.