Mason Brown, alias Bruce Brown in the Japanese iteration, serves as a Subterra brawler within the Gundalian Agents, deployed to infiltrate Bakugan Interspace during the Neathian-Gundalian war. Though outwardly compliant with Emperor Barodius’ regime, his true devotion resides with Gundalian elder Nurzak, a mentor figure commanding unwavering respect. His fractious partnership with Subterra Avior, a guardian Bakugan he derisively nicknames “old timer,” fuels constant bickering—Avior chastises Mason’s cocky demeanor, while Mason dismisses his partner’s warnings as nagging.

Prone to theatrical displays and maintaining icy composure mid-battle, Mason’s patience frays in downtime, often scapegoating Avior for mishaps. His loyalty to Gundalia coexists with open admiration for Neathian warrior Fabia Sheen, whose advanced Battle Gear tactics earn his begrudging praise. Their encounters spark playful flirtations, swiftly countered by Avior’s reminders of their mission parameters.

Tasked with evaluating brainwashed recruits in Bakugan Interspace, Mason clashes with Dan Kuso and Jake Vallory, suffering a rematch defeat when Julie Makimoto tilts the odds. Evading capture after Kazarina’s ship falls to the Twelve Orders, he joins Ren Krawler’s strategic operations before the Sacred Orb’s dimensional surge strands him alongside Nurzak and Sabator. Post-Dan’s victory over Dragonoid Colossus, he aligns temporarily with the Battle Brawlers, aiding in liberating brainwashed allies and confronting Barodius’ forces.

Collaborating with Fabia and Nurzak to thwart Kazarina and Gill, Mason later scours battlefields seeking Nurzak after his apparent demise. During the war’s climax, he duels Phantom Dharak alongside former teammates before falling in combat. Post-conflict, he retreats to Gundalia, resurfacing later to repel Chaos Bakugan invasions during Mechtanium Surge alongside Avior and Jesse Glenn. His arsenal grows with Subterra Crashbor, a Mechtogan, and Subterra Fortatron Battle Suit, though neither sees pivotal use.

Mason’s survival instincts spare him the Twelve Orders’ retribution meted upon peers like Lena, Zenet, and Jesse. Evolving from comic antagonist to shrewd survivor, his priorities pivot from blind Gundalian fealty to pragmatic alliances, anchored by loyalty to Nurzak. While respecting Fabia and the Brawlers, he maintains Gundalian allegiance, balancing contentious camaraderie with Avior against battlefield efficacy—evidenced by their coordinated escape from Sacred Orb-induced dimensional displacement.

Combat records highlight calculated victories against unnamed Ventus adversaries and a staged defeat with Lena to gauge opponent strength. Losses stem primarily from protagonist interference or external factors, underscoring a tactician’s prowess tempered by circumstance.