Reugosite

Reugosite (ルーゴサイト) is the final villain of Ultraman R/B. It is stated to be a dreadful life form that brings catastrophe to the universe.

Subtitle: Cosmo Eater (コスモイーター)

Pre-Ultraman R/B
Reugosite was a planet-eating monster that destroyed everything in its path. 1,300 years ago, It came across Earth and was met with resistance in the form of three warriors sent from Planet O-50; Ultraman Rosso, Ultraman Blu, and Grigio Bone, the latter being transformed into by Saki Mitsurugi. Reugosite attempted to destroy them and managed to knock them onto Earth, resulting in the Ayaka Comet incident. After that, Reugosite seemingly left.

Ultraman R/B
As Aizen Tech detected that Reugosite would be returning to Earth, Saki plans to turn the planet into a massive bomb in an attempt to destroy the powerful monster. Having returned to Earth in present times, Mio Minato suceeded in imprisoning Reugosite into a R/B Crystal, using equipment from Aizen Tech and the three Gyros. She later wanted to have the R/B Crystal thrown into the dimension she was trapped in for the past 15 years. However, Saki claimed the crystal after taking back her Gyro, closing the portal and stating that Mio's way is not the solution in stopping Reugosite. After Saki defeats Ultraman Ruebe as Grigio Regina, Reugosite's R/B Crystal activates itself, cancelling out her transformation and assuming its true form via Saki's Gyro.

Reugosite proceeded to destroy everything in its path until Grigio Regina reappeared. Saki tried to stall Reugosite until the Ray Energy gathered at the Aizen Tech premises, while Reugosite withstood all her attacks. Once that happened, a barrier was placed around Aizen Tech to prevent Reugosite from escaping. The Aizen Tech building's Monster Restraint System was also fired at it, but Reugosite shrugged it off and used its tentacles to cut off the part of the tower that fired the beam. Preparing to finish off Grigio Regina, Reugosite fired the Genesis Requiem, which was blocked by Rosso and Blu, who appeared and fired their signature Ultra Beams to counter it. Reugosite fired yet another beam and it hit Grigio Regina, who defended the brothers after being reminded of what her brothers did for her 1300 years ago, and Saki died doing so.

Rosso and Blu then charged in fighting and trnsformed into Ruebe. During the battle, electrical equipment was damaged and the barrier around Aizen Tech was shut down. Just as Rosso and Blu were about to be finished off by Reugosite, Mio was reminded of what she saw in the other dimension and cut off their transformation by locking her sons' Gyros. The Aizen Tech tower fired another beam that momentarily stunned it. Meanwhile, Mio prepared the final step of ther plan, which was to pull the entirety of the Aizen Tech premises along with Reugosite and herself into a giant portal to the other dimension. After a talk among the Minato family, the brothers transform once more. Rosso, Blu and later on Ruebe fought well, convincing Mio to stop and believe in her sons. Despite that, Ruebe's power was not enough and was almost defeated, until Asahi realized what she truly is and transformed into the Makoto Crystal. With a final beam lock between the Genesis Requiem and the Shin Vortex Buster, Reugosite was finally put to an end.

Trivia

 * Suit Actor: Shinnosuke Ishikawa
 * Reugosite's concept art from the toy catalog has been compared to Greeza by fans.
 * Its name is one kana away from the Japanese transliteration of Leukocyte (ルーコサイト), the technical term for white blood cells. Mio Minato also makes this allusion, saying that Reugosite is like the white blood cell of space that has determined that Earth is like a virus that should be annihilated.