Oz Mare serves as an operative in the First Unit of Dorothea, a government agency regulating illegal fairy usage. He is a tall, wide-shouldered, bald middle-aged man who wears the standard Dorothea uniform.

During operational duties, Oz observed a training session where he expressed admiration for Marlya Noel's rifle marksmanship. He later attended a welcoming party for Marlya hosted by agent Serge Tova at a bar, socializing with fellow Dorothea members.

A critical mission tasked Oz's unit with transporting a fairy weapon for Duke Schwarz Diese. Their train was ambushed by Beevee Liscar and his forces. Oz engaged Liscar in combat using his bull-like fairy but was swiftly overpowered. Seeing Sophie poised to attack Marlya, Oz intervened, shielding her. He carried Marlya away from immediate danger but sustained multiple gunshot wounds to his back. Despite his injuries, he continued protecting Marlya by covering her body with his own. Liscar approached the fallen Oz, remarked that his injuries would prevent further fighting, and delivered a fatal blow, claiming to "save" him from that fate. Oz died maintaining his protective stance over Marlya, smiling at her with his last breath as grieving Dorothea colleagues surrounded them.

Following his death, Free Underbar kept Oz's arm patch while grappling with survivor's guilt. Marlya internalized his death as confirmation of her self-perceived status as a "cursed child," leading to emotional withdrawal. The event prompted collective mourning among Dorothea members, significantly impacting unit cohesion and morale.