Mozgus is a chief inquisitor of the Holy See, serving as the primary antagonist during the Conviction arc. He is a tall, pale-skinned man with blue eyes and a notably smooth, flattened face, a physical deformity resulting from years of violent prostration during prayer, a practice that also causes his face to become veined when he is angry or praying fervently. He wears a white and gold ecclesiastical vestment with a red cloth bearing the insignia of the Holy See, and a wide-brimmed red hat with ear flaps. His personal symbol is four broken torture wheels, representing his brutal methods.

His background within the Holy See is one of complete devotion and institutional advancement. From an early age, he was formed within the church, developing an absolute faith in its doctrine and his authority to enforce it. He became the most feared and famous inquisitor, traveling with a group of personal torturers and bodyguards whom he recruited from the margins of society. These disciples were outcasts, disfigured or ostracized individuals whom Mozgus rescued and gave purpose, telling them that their deformities were gifts from God and training them in his methods of torture. He cared for them in his own way, becoming deeply distressed by their deaths.

Mozgus's personality is defined by a complete merger of his individual consciousness with ideological certainty. He is not a hypocrite who uses religion as a mask for sadism; he genuinely believes that his brutal actions serve divine will. He tortures not despite moral doubts, but with the sincere conviction that suffering is a righteous and necessary form of correction for heretics. He is utterly indifferent to the humanity of his prisoners, viewing them only through the lens of theological transgression. He is highly sensitive to anyone who claims to speak for God, and he has killed a villager for suggesting that God might punish Mozgus for his arbitrary cruelty. Despite the apparent sadism, he demonstrates a grim awareness of the sorrow his work causes, but he feels no guilt, believing that spilling blood is a path to a devout existence.

His primary motivation is the purification of heresy through any means necessary. He sees defiance against himself as defiance against God, and he believes that his inquisitorial authority makes him the embodiment of divine rule. He is sent to the city of Albion to root out a heretical cult, but he focuses most of his energy on violently suppressing the starving and rebellious population, viewing their desperate acts as sin. He uses the Holy Iron Chain Knights, including Farnese, as his personal guard, and his methods deeply impress Farnese with their harshness and apparent effectiveness. A significant event illustrating his warped sense of mercy occurs when he spares a young mother begging for food for her child: he gives the child food and medical attention, but sentences the mother to torture, tearfully praying she survives the test God has prepared, using the incident as a lesson for Farnese about the harshness of divine doctrine.

In the story, Mozgus serves as the central human obstacle to Guts's mission to rescue Casca. He is sent to Albion to suppress heresy but becomes fixated on Casca after she is accused of witchcraft. He intends to torture her for information about the Black Swordsman, but his actions inadvertently trigger the appearance of a massive band of demons, drawn by the Tower of Conviction and Casca's Brand of Sacrifice. When supernatural chaos erupts, he does not flee; instead, he retreats to the chapel of the Tower of Rebirth, where an Apostle-Behelit transforms him and his disciples into pseudo-apostles with angelic features. Mozgus, never realizing he has been corrupted, believes his new wings, strength, and regenerative hide are divine gifts. He then decides to burn Casca at the stake to both stop the ceremony and force Guts to confront him, seeing himself as an angelic warrior of God.

His key relationships are primarily with his disciples, whom he rescued and commands with paternalistic cruelty, and with Farnese, to whom he acts as a harsh mentor, teaching her about the merciless application of faith. His relationship with Guts is one of direct antagonism, with Mozgus viewing the Black Swordsman as a heretic to be destroyed. His development is a descent into monstrous form that mirrors his spiritual corruption. He begins as a human fanatic and, after transformation, becomes a winged, armored demon whose appearance reflects his ideals of religious purity, with hard, scale-like feathers and a body encased in a hide that can withstand powerful blows. The only weak point in his transformed state is a plate bearing the insignia of the Holy See, representing the book of scripture he always carries. This detail symbolizes that, despite his power, his vulnerability is tied to his absolute faith. His final defeat comes when Guts drives his sword through this weak point, and his death is followed shortly by the completion of the Incarnation Ceremony and the destruction of Albion.

As an inquisitor, his abilities originally centered entirely on institutional authority, psychological manipulation, and expertise in torture. He was a skilled interrogator who could manipulate crowds and individuals through fear and theological rhetoric. After his transformation into a pseudo-apostle, he gained supernatural strength and durability, wings that allow flight and can be used as additional arms for powerful punches, and a scaly hide nearly impervious to normal weapons. In his fully transformed state, his body becomes completely enveloped in hardened feathers, giving him a bladed, armored appearance, and he can use an attack that creates a shockwave by slamming his wing-arms into the ground. His fight with Guts is relentless, and he nearly defeats the Black Swordsman before his weak point is exploited.