Aum is a character from the Skypiea arc, serving as one of the four holy priests who act as the direct subordinates of the self-proclaimed god, Enel. Known by the title Sky Breeder, Aum is responsible for administering the Ordeal of Steel, a trial with a zero percent survival rate, to those who trespass into the sacred grounds of Upper Yard.

Aum’s personality is defined by a cold, fanatical devotion to his duty and a deeply twisted sense of mercy. He frequently laments the lives of his opponents, expressing a sorrowful belief that by killing them, he is performing an act of salvation and freeing them from the suffering of their mortal existence. This detached and philosophical cruelty makes him a formidable and unsettling adversary. His motivation is rooted in his absolute loyalty to Enel and his rigid adherence to the laws of Skypiea, viewing the punishment of criminals not as violence but as a divine duty.

Within the narrative of the Skypiea arc, Aum acts as a major obstacle for the Straw Hat Pirates, specifically challenging Roronoa Zoro. He guards a crucial path on Upper Yard, and his combat style forces Zoro to adapt and overcome a unique set of challenges. A key relationship is with his giant pet dog, Holy, a highly obedient companion trained in martial arts who fights alongside him. While Aum commands Holy with authority, the dog also demonstrates a comedic and notable weakness, becoming friendly and obedient to strangers when offered praise, a trait that Zoro exploits during their battle. Another significant relationship is his service to Enel; like the other priests, Aum possesses the ability to use Mantra, the Skypiean term for Observation Haki, which allows him to predict his opponents' movements. This power contributes to his arrogance and the seemingly insurmountable nature of his ordeal.

Aum shows little to no character development throughout his appearance, remaining a static representation of Enel’s oppressive and merciless rule. He is eventually defeated by Zoro, who manages to cut him down despite his Mantra-predicted defenses by launching a powerful attack through a solid wall, demonstrating that sheer will and power can overcome premonition. His notable abilities are centered around his unique weapon, a large blade made of iron cloud. Using a specialized Dial embedded in the sword, he can manipulate the metal's shape at will, transforming it into various forms for attack and defense, including a whip, a shield, a fan, a fork, and a long thrusting sword. This versatility, combined with his Mantra and his partnership with Holy, makes him one of the most technically skilled and dangerous warriors in Skypiea.