Nillious is a multi-headed dragon-like or hydra-like Bakugan who appears in multiple generations of the Bakugan anime. In the Battle Planet series, he is a Darkus Bakugan who serves as the partner of Magnus Black and acts as the primary rival to Dan Kouzo’s Dragonoid. In a later reboot series, he is a chimeric snake-dragon Bakugan and the true main antagonist of the first season, originally partnered with Kage before defecting and later allying with V. He originates from the Vestroia Star System, and before arriving on Earth he commanded an army with an iron fist. His aggressive policies and use of experimental weapons caused a rift with another commander named Drago, which ultimately led to a tear in space-time that brought the Bakugan to Earth.

Nillious is characterized as vicious, untamable, and driven by a desire to inflict harm rather than to win brawls. He dislikes fighting as part of a team and prefers to act on his own, satisfying only his own impulses. Each of his heads has its own personality and abilities. In the Battle Planet series, his main motivation is to kill, not to win. In the reboot series, he holds the belief that Bakugan should remain within their own clans and opposes groups like the Misfit Clan. He abandoned Kage after the latter demanded respect, later choosing to align with V because he considered V a powerful brawler.

Nillious plays a significant role as a recurring antagonist. In the Battle Planet series, he is the guardian Bakugan of Magnus Black and a constant rival to Dragonoid, evolving multiple times over the course of the story. In the reboot series, he is the main antagonist of the first season, a former army commander who becomes a rogue force. His key relationships include his human partner Magnus Black, with whom he has a tense dynamic as Magnus struggles to keep him in line, and his rivalry with Dragonoid. In the reboot series, he is also the archenemy and evil counterpart of Drago.

Nillious undergoes considerable visual and power evolution. In the Battle Planet series, he first appears as a two-headed hydra-dragon with mechanical wings. During the Armored Alliance season, his wings become more organic, and in Geogan Rising he gains a third head and a centaur-like lower body. By the Evolutions series, he becomes an Elemental Bakugan. In later seasons like Bakugan Legends, he returns to a two-headed bipedal form in his Nova mode, though the Nova Strength form retains the three-headed centaur design. In the reboot series, he is depicted as a two-headed dragon with sharp claws, talons, and a split-barbed tail, with a slim build and dark purple forked tongues. His abilities include powerful techniques such as Shadow Breath and Pact of Darkness, and he is noted for being stronger than average, able to fight multiple opponents at once. Each head’s independent personality also grants him tactical versatility in battle.