Magnus Black is a human male character who first appears as an antagonist in the anime Bakugan Battle Planet before evolving into an anti-hero across subsequent seasons. He is fourteen years old during the events of Battle Planet, Armored Alliance, and most of Geogan Rising, though a specific incident later regresses his biological age to no more than twelve.

Magnus grew up in poverty with his younger sister, Emily, forming an exceptionally close bond with her. When Emily fell ill with a rare and mysterious disease, Magnus took responsibility for her care. To secure the life-saving technology and medicine needed for his sister, he accepted an offer to work for AAAnimus Inc., a corporation that promised to provide her medical treatment in exchange for his services. His mission involved defeating the Awesome Brawlers and capturing their Bakugan.

In terms of personality, Magnus is enigmatic, dark, and typically reserved. He becomes easily frustrated when provoked by excessive talking or annoyance. He keeps his identity a mystery and has no interest in fame or recognition. Despite working for AAAnimus, Magnus strongly believes in the bushido code of the samurai, treating Bakugan battles as serious endeavors rather than trivial games. However, he has clearly lost his way in applying this code. Part of Magnus feels ashamed of his actions on behalf of AAAnimus, which is why he hides behind a mask and adheres to a strict code of honor in the field. Beneath his cool and distant exterior, he has a soft spot for his sister, and his dedication to brawling stems entirely from his desire to save her rather than for personal enjoyment.

Magnus serves as the primary rival to Dan Kouzo, the protagonist of the series. He takes an instant dislike to Dan because he recognizes that Dan possesses a special, noble bond with his Bakugan partner, Drago. This jealousy fuels Magnus's drive to become the best battler possible, dedicating himself to defeating Dan to prove his own worth and ultimately save his sister. Armed with a cadre of powerful Bakugan, Magnus is an exceptionally skilled battler, considered the best except for Dan. His primary rival is Dan Kouzo, but he also interacts with other members of the Awesome Ones, including Shun Kazami, who once foiled Magnus's plan to infiltrate their group by outbrawling him due to Magnus's reckless tactics.

Magnus's development across the series is significant. He begins as a villain working for AAAnimus, following their directives without question. However, after his defeat by Dan, he leaves the organization and adopts a vigilante role, actively pursuing criminals who exploit Bakugan power for their own purposes. He later enrolls as a teacher at Bakugan Academy under the alias Mr. Magnus, conducting an undercover investigation into unauthorized elemental evolutions and unethical experimentation at the school. During this time, he mentors students, including Dan, helping them unlock advanced techniques such as Speed Mode. Throughout these transformations, Magnus's actions remain consistently motivated by his desire to protect his sister Emily. Over time, he reconciles with Dan and collaborates with the Awesome Brawlers against larger threats.

Notable abilities include Magnus being a Darkus attribute battler whose Guardian Bakugan is the dragon-like Nillious. He also owns other Darkus Bakugan, including Fangzor, Webam, and Eenoch. In later seasons, he utilizes subspecies such as the Darkus Geogan Hyenix and the Nanogan Widow. He was the first brawler to evolve a Bakugan in Battle Planet and the first to defeat an Aurelus Bakugan. Due to being born before the Great Collision, he requires his gauntlet to brawl. Nillious itself undergoes multiple evolutions, from a two-headed hydra-dragon to a three-headed form, representing Magnus's growth and the deepening bond with his partner.