Rubedo (a.k.a. Gaignun Kukai, Jr.) is an U.R.T.V. (U-DO Retro Virus) that was created to combat U-DO, which had appeared on the planet Milta in T.C. 4740. He had the ability to keep his fellow U.R.T.V.'s (which included Albedo Piazzolla and Nigredo, a.k.a. Gaignun Kukai) through a spiritual link, Rubedo suddenly had a vision of Milta's destruction, which broke the link and caused the U.R.T.V.'s to go mad and slay the residents of Milta, as well as the other U.R.T.V.'s. Rubedo, along with Nigredo, were rescued before Milta disappeared into the Abyss by Jin Uzuki, the older brother of Shion Uzuki, and chaos.

Although Rubedo is the oldest of the U.R.T.V.'s surviving, he has had his body modified to remain at its appearance of a 12 year-old boy. Because of this, he has taken the guise of the adopted son of Gaignun Kukai (Nigredo). He also has the ability to enter a special mode (Red Dragon), which he can use to combat U-DO. However, sometimes it is hard to tell if he is a mature being or still a little boy, since he can still act a little rash.

Rubedo also has access to the Kukai Foundation's assets, which include the dreadnought Durandal, which he uses to collect and confine the 13 Zohar Emulators which were created. He also uses the Durandal to confine humans that were turned into Gnosis by the touch of other Gnosis (Shion Uzuki is the one human who has escaped this awful fate). He is also very fond of MOMO, because of his previous relationship to the person she was modeled after, Sakura Mizrahi.

(source: Absolute Anime)