The Grongi are an ancient civilization consists of beings that can transform into monsters to kill people for their ruthless game, which is translated into the Grongi Language as the "Gegeru". Attacking their rival civilization the Linto, the original Kuuga defeats the leader of the Grongi and seals all 200 of the Grongi away. Also protecting the Linto were the Lords, who would be the primary villains of the sequel to Kuuga, Agito. However, by present day, the newly resurrected Daguva revives his tribe from their rest and resume the Gegeru on the human race, the descendants of the Linto, to bring out the "Ultimate Darkness", where humanity will be as violent as the Grongi. They are referred by the Police as Unidentified Life Forms. Each of them possesses a strange artifact in all of their bodies, no different to Kuuga. Those seen to assume human form are labeled as "B", each the human-like Grongi possessing a tattoo that resemble their true animalistic forms. Unidentified Life Form #B1 only remains in human form for the rest of the series. Each Grongi has a stylized belt buckle in their true form which if destroyed causes the Grongi's destruction, as seen when one is hit with one of Kuuga's finishers, causing cracks to spread out from the site of the impact until it reaches their belt buckle, upon which light pours out of it and the Grongi explodes. The naming pattern of the Grongi is "Group-Species-Creature type".

N-Daguva-Zeba is the stag beetle-like lord of the Grongi Tribe responsible for murdering the archaeologists before resurrecting the Grongi in modern times, his Imperfect Form labeled as "Unidentified Life Form #0" by the Police. Before resurfacing nine months later in his fully restored Perfect Form, Daguva manages to slaughter 152 inferior Grongi in Nagano before killing Gooma and falling off the radar after six days of being sighted. By the time he resurfaces, Daguva assumes the guise of a white-suited young man with an aura and is labeled as "Unidentified Life Form B13". Once Gadoru is killed, Daguva begins end game by causing an endless heavy rain before defeating Kuuga in his Amazing Mighty Form while leaving massive damage in his wake. As he revealed to Baruba, he is giving Kuuga time to get stronger as a favor for giving him time to move around the country in peace. Once at Nagano, Daguva resumes the carnage before starting a systematic genocide of all humans in Japan as he tells Yuusuke to meet him at Mount Kuro for a final confrontation. But as he hoped, Daguva assumes his Grongi form as he and Kuuga battle each other. After exhausting themselves to the point that they are too weak to maintain their fighting forms, they continue to fight as humans with their bare hands to the death until Daguva dies from the multiple fist injuries.
Ra-Baruba-De is the first Ra to appear, assuming the guise of a Rose-Tattooed Woman, she is labeled "Unidentified Life Form B1" and served as the judge for Zu and Me group Grongi, as well as bringing Go into the game before making visits to Nagano. Though glimpses of her true form were seen, she never assumed it before any human and was believed by Ichijo to be #0. However, when she realizes how far humans have changed after being wounded by Ichijo, Baruba hints the Kuuga/Daguva relation before she hospitalized the detective for being able to actually hurt her and starts to warn the other Grongi to no longer underestimate the humans. During Daguva's attack, she is encountered by Ichijo as she reveals that humans will soon be "like the Grongi" before running off, with Ichijo forced to use 8 Nerve-Breaking Bullets to take her down as her lifeless body falls into the sea and is unable to be found.

Armed with a pair of tonfa, Dorudo assumed the form of a man in black with white robes on him. He carries an abacus which he uses to count the number of kills by the Go Group Grongi until Ichijo destroyed it with a Marking Bullet, forcing the Grongi to reveal his true form in a hit and run attempt to kill Ichijo. However, as a result, Dorudo lost his purpose as the game's overseer and is forced to fight for his life against Gadoru as Ichijo arrives to the scene. Dorudo eventually attempts to retreat from the fight and is pursued by Sugita and Sakurai who shoot him dead with the completed Nerve-Breaking Bullets.
Nu-Zajio-Re is the last of the Nu and is labeled "Unidentified Life Form B14", the group within the Grongi who crafts the artifacts used by the other groups. Posing as middle aged man, Zajio manages to remain in hiding with his existence unknown to the police while creating the items used by the Grongi. With Gooma's aid, Zajio obtains discarded pieces of Daguva's body and refines them to create a means for a Grongi to have the same abilities as Daguva and Kuuga, so those in the game can defend themselves against Daguva. In the end, Zajio is eventually killed by Daguva just as he finished his final work. Like Baruba, Zajio's true Salamander form was never revealed.
Be-Jimin-Ba is a water flea Grongi who is one of the lowest Be Group, whose others members were all murdered by Daguva without acquiring the right of the game. The monster only appears on the Kuuga stage show.
Series:  Kamen Rider Kuuga