Bull Armor, originally named Fighting Bull, was the star quarterback for the Great Gigantes roughly ten years before the main events. During a pivotal game, he activated full power mode to retaliate against opponents who injured his teammates. This triggered a program bug, causing him to indiscriminately attack allies and rivals alike. He subsequently retired from the Iron League and took a secret position as a hospital nurse, driven by a desire to atone for his actions. He grappled internally with a longing to return to sports and lingering guilt over his past.

His hesitation ceased after an encounter with Magnum Ace, who inspired his return to competition. As an American football specialist, Bull Armor possessed unique athletic capabilities, including being the only player who could catch Magnum Ace's 44 Sonic pitch. On the team, he primarily served as the goalkeeper in soccer matches, leveraging his strength in crucial defensive roles.

Bull Armor displayed an inherently kind and caring personality. His compassion extended beyond sports, notably demonstrated when he used his strength to rescue a child from an oncoming truck. He often expressed himself with vocalizations like "Dorya~!" and used his horns as emotional indicators: they pointed downward or backward when he felt reserved, nervous, or scared; upward during calm, happy, or determined states; and forward when fully energized or in combat-ready modes.

Memories of his berserk incident sometimes surfaced, leading to moments of intense self-doubt or hesitation. However, his commitment to protecting others and supporting his teammates remained unwavering throughout his development.