U-DO (pronounced OOH-doo) is a wave existence that is directly connected to the Miltian Conflict and the Zohar. It is a collective consciousness with a single mind, unlike the human collective consciousness with individual minds. It is the quality of having a single mind that makes U-DO's collective far more powerful than the human collective. It is this infinite energy that humans have both feared and tried to control. U-DO dwells in the higher domain, observing humankind in the lower domain with its observation terminals, also called its "eyes," Abel and Abel's Ark. Abel is the human incarnation of U-DO in real space (the physical world), and Abel's Ark is Abel's imaginary (spiritual) form in real space, which is why it is mistaken for a Gnosis. U-DO is also sometimes referred to as "God" by both Dmitri Yuriev and Wilhelm; however, this term is also applied to chaos, which makes it unlikely there is a single God, rather higher beings who are seen as such or different aspects of God. Although if chaos is the God of the Lower Domain's universe, it might be said that U-DO is the God of the Higher Domain's universe.
U-DO is an acronym that stands for the Unus-Mundus Drive Operation system. Dmitri Yuriev, father of the U.R.T.V. units, and others who were studying U-DO told the government that U-DO was an artificial consciousness or A.I. linked to the U.M.N. that was carrying some kind of dangerous waves. That was a complete lie, told in order to hide the fact that U-DO was something Dmitri Yuriev did not entirely understand. U-DO is actually an energy from a higher dimension. According to Junior, U-DO is an entity humans could not possibly control. It appears as a huge cloud of reddish-colored gas with long tendrils and can create space-time anomalies.
The U.R.T.V.'s are specially designed to combat U-DO; they are U-DO's anti-existence. U.R.T.V.'s can produce anti-waves designed to resist U-DO, but these anti-waves will not neutralize each other. The waveform collision between these anti-waves will release enormous amounts of thermal energy, resulting in the destruction of anything near them, including the U.R.T.V.'s. A moment before the U.R.T.V.'s were to destroy U-DO, Rubedo had a vision of the Old Miltian planet being destroyed in an enormous explosion. Rubedo, fearing for the lives of his companions, broke the psychic bond that would produce the anti-waves. Unfortunately, this left them with no protection against U-DO. U-DO slew or drove mad all of the standard U.R.T.V.'s. It then infected Albedo and drove him mad as well (although he already was a little insane before that). The only documented surviving U.R.T.V.'s were Nigredo and Rubedo, though Albedo survived as well.
When Albedo made partial contact with U-DO fourteen years prior to Episode I and II's setting (when he was infected by U-DO), it caused a medium-scale space-time anomaly alteration that produced two black holes. The planet of Old Miltia (back then, simply called Miltia) was sucked into those black holes and into a place called the Abyss (possibly just an empty vacuum within the black holes). Since then, U-DO had been obstructed by the two black holes. Fourteen years after Old Miltia's disappearance, Albedo obtained the Y-Data by hacking into MOMO's systems and used it to bring Old Miltia back from the Abyss, sacrificing himself in the process. After this occurred, the two black holes vanished.
Joachim Mizrahi developed a way to use U-DO as a control system for linking to the Zohar. Also, Dmitri Yuriev used Abel to pilot Ω Res Novae. Abel was the only one who could withstand the construct's awesome power without going mad.
U-DO has some speaking lines in Episode III; it is voiced by Doug Erholtz in the English version and by Hikaru Midorikawa in the Japanese version. Interestingly, Hikaru Midorikawa also voiced Fei Fong Wong from Xenogears, meaing that the voice of U-DO would also be the voice of Abel in his adult form.