Mugen, a small red dragon adorned with white markings across his stomach, tail, and extremities, sports a signature scarf. Exiled on his homeworld for refusing a human partner, he adopted a rebellious, lone-wolf persona akin to a Japanese delinquent until Rei acknowledged him as the "number one bright red" dragon. This recognition forged their partnership—a bond marked by fierce loyalty and frequent clashes, reflecting Mugen’s brash arrogance against Rei’s laidback demeanor.

As the spirit card Mugendra, he shifts into card form during combat, acting as Rei’s frontline ally. His fiery impulsiveness extends to unconventional tactics, including weaponized flatulence. This comedic yet potent skill, dubbed "Mugen’s Flame," scorches foes like the Triumvirate.

A fervent admirer of Magical Star Saki, whose likeness to his childhood friend Stardra fuels his obsession, Mugen drains the crew’s finances by impulsively purchasing a rare Magical Star figure. His determination to restore a stolen ribbon to her reveals a sincere core beneath his cocky exterior.

Evolving through battle, Mugen ascends from Mugendra to Mugendragon, then Mugendragon-Nova during a clash with Hansoro. His ultimate form, Ultimate-Mugendragon, emerges in the fight against Denebola, enabling him to strike decisive blows in pivotal moments, such as Rei’s duel with Nakes.

Once isolated and rejected, Mugen matures into a strategic protector through his bond with Rei. As the first fully realized spirit character within Battle Spirits lore, he operates autonomously in gameplay despite his non-human status. Temporary corruption by Ultimate-Siegwurm-Nova tests him, but Rei’s summons restores his resolve.

His rivalry with Ian, Kiriga’s composed dragon partner, pits fiery recklessness against icy calm. Interactions with the April siblings and robot Salt oscillate between rowdy humor and heartfelt moments, cementing Mugen’s dual role as both the crew’s prankster and emotional cornerstone.