Rossiu Adai is one of the members of Team Dai-Gurren who later becomes Chief Adviser of the Human Government and one of the main characters in Tengen Toppa Gurren Lagann. Despite serving as a protagonist in the "Beastmen War" arc, Rossiu becomes an antagonist, with his attempts to maintain peace and order in human society in wake of attacks from the Anti-Spiral leads him to perform a coup and throw Simon in jail under false charges.
He was voiced by Mitsuki Saiga in the Japanese version, and Johnny Yong Bosch in the English dubbed version.
Rossiu originated from the village of Adai, an underground society who worshipped Gunman as gods. After the twins, Gimmy and Darry, were chosen to leave the village to prevent overpopulation, Rossiu decided to go with them out of a desire to see what the surface was like.
After an encounter with Simon and Kamina, Rossiu joined Team Dai-Gurren to fight against the Beastmen and later became the pilot of Gurren after Kamina died in battle against the Beastman General Thymilph the Raging Wave.
After Team Dai-Gurren took Teppelin and defeated the Beastmen Army, Rossiu became a leading member of the new Human government set up, serving as Chief Adviser to Supreme Commander Simon. Things remained peaceful for nearly a decade, until a group of humans desired to return underground. Rossiu blocked the humans from returning to the underground as it would be harder to keep track of their growth if they weren't on the surface, remembering Lordgenome's final words that the moon would crash into the Earth if the human population reached one million. In response to the measure, conflict broke out, with a rogue group of Beastmen led by Viral attempting to aid the humans in returning to the underground, and a Grapearl squadron led by Simon in Gurren Lagann was sent in to take them out.
Despite Viral's insurgency being quashed, unrest continued to rise. After the millionth child was born, triggering the Anti-Spiral's Human Extermination System to deploy Mugann, riots began to break out against the government, blaming Kamina and Simon for the destruction wreaked by the Mugann. Seeing no other option to keep the peace, Rossiu performed a coup against Simon and held him as responsible for mishandling the Mugann attacks, taking him into custody. Rossiu set up a kangaroo court to try Simon and had him incarcerated, usurping his position as Supreme Commander. Rossiu hoped that this decision would restore order but unrest continued to rise.
Eventually, after the moon was set by the Human Extermination System to collide with Earth, Rossiu had the human population evacuated onto the Ark Dai-Gurren, planning to head out into space to find a new planet to colonize and restore their civilization. However, as the Ark Dai-Gurren was departing from Earth, the spacecraft fell under attack from Mugann squadrons. Before they could be destroyed, Simon and Viral in Gurren Lagann, alongside the rest of Team Dai-Gurren, showed up and destroyed the Mugann. Simon managed to stop the moon from colliding with Earth and revealed it to actually be Lordgenome's old ship before pulling the real moon back into place.
Rossiu then realizes how misguided his actions were and how they were all for nothing. Out of despair for betraying his friends, Rossiu attempts to commit suicide using a handgun and leaves a message behind expressing remorse for his mistakes. However, Rossiu is stopped by Simon and Kinon, with Simon teleporting to his location and punching him in the face before he could pull the trigger. Simon then gives Rossiu the encouragement Kamina once gave him and snaps Rossiu out of his depression.
As Team Dai-Gurren left to go defeat the Anti-Spiral and liberate the universe, Rossiu stayed behind on Earth to lead the human government while Simon was away. After Simon left following the destruction of the Anti-Spiral and Nia's wedding to walk the Earth, Rossiu became Supreme Commander once more, a position he is still shown to hold in the epilogue.