Rossiu hails from Adai Village, an isolated underground settlement that worshipped Gunmen as divine entities and enforced strict population control through exile. His mother was among those expelled, resulting in her death on the surface—an event that deeply shaped his views on sacrifice and survival. When village elders chose the twins Gimmy and Darry for exile, Rossiu accompanied them to the surface, driven to verify its existence.

During the Beastmen War, Rossiu encountered Kamina, Simon, and Yoko. Initially distrustful, he staunchly defended Adai's religious doctrines until the village elder piloted their sacred Gunman, revealing the belief system as a fraud. This shattering revelation led Rossiu to join Team Gurren alongside Gimmy and Darry. After Kamina's death, he assumed control of the Gunman Gurren, showcasing tactical skill in battles like evading Cytomander's missile barrage. His analytical nature contrasted sharply with Team Gurren's impulsiveness, cementing his role as a strategic asset in the campaign against Lordgenome.

Seven years after the war, Rossiu rose to Chief Advisor in Kamina City's government. Learning from Lordgenome's memories that humanity reaching one million would trigger the Anti-Spiral's Human Extermination System, he enacted restrictive policies to monitor population growth, including blocking attempts to return underground. When the population threshold activated the Anti-Spiral attacks, he orchestrated a coup against Simon, imprisoning him under false charges to suppress civil unrest and consolidate control. His contingency plan involved evacuating half of humanity aboard the Arc-Gurren spacecraft, condemning the remainder to perish during the moon's collision with Earth—a decision influenced by Leeron's calculations confirming planetary uninhabitability post-impact.

After Simon halted the moon's descent and rescued the Ark-Gurren from a Mugann ambush, Rossiu recognized the futility of his actions. Overwhelmed by guilt over betraying his comrades, he returned to Adai Village intending suicide. Simon and Kinon intervened, with Simon delivering Kamina's iconic phrase—"Don't believe in yourself. Believe in me who believes in you"—to dissuade him. This confrontation halted Rossiu's self-destructive path, prompting his recommitment to responsible leadership.

In the epilogue set twenty years later, Rossiu serves as President of the post-war society, overseeing galactic peace initiatives. He acknowledges Simon could have achieved comparable governance but respects his choice to live anonymously. Rossiu's administrative approach balances Spiral energy advancement with ethical constraints, reflecting his reformed perspective on progress and sacrifice. The film adaptation *The Lights in the Sky Are Stars* streamlines his antagonistic actions, omitting extreme dialogue and emphasizing his pragmatic motivations over outright villainy while retaining his core character trajectory.