Yuriev
Dr. Dimitri Yuriev is the head of the Yuriev Institute. His life is dedicated mainly to the study and destruction of U-DO. He was the genetic father of all the U.R.T.V. units, and so of Citrine. He was killed by Nigredo because his son no longer wanted to complete his mission of killing the Red Dragon, it being in fact Rubedo, and Gaignun was created to kill Rubedo in case his unstable powers flared out of control. Yuriev will latter come back and take over Gaignun's body in order to achieve his special plan, to become a God.
Citrine served the same purpose as Nigredo, to kill Red Dragon. Also, she was raised following his order to become one of his best agent.
Rubedo
Rubedo, often called "Little Master" by those under his command, who named himself Gaignun Kukai Jr., is a director of the Kukai Foundation, an organization based on Second Miltia. He looks like a child, but is in fact one of the variants URTV, #666, having grown up in the Yuriev's Institute, and being Citrine's older brother. He defeated Yuriev's plan on the Old Miltia by not following his orders, and showed sign that he carred for Citrine. Nevertheless he'll have to be the one to take her life when they'll meet 15 years latter.
Albedo
Albedo was created by Dmitri Yuriev as U.R.T.V. Unit # 667, originally a conjoined twin with Unit #666, Rubedo. stands for U-DO Retro Virus, he went mad after having contacted U-DO, and became one of the main antagonist as adult. He never had any real interraction with Citrine, outside meeting her along his two brother at the Institute, then mentionning she may spy on them along with Nigredo after both of them died.
Nigredo
Nigredo is the chairman of the Kukai Foundation under the alias of Gaignun Kukai, a special organization based in Miltia, he is a genetically altered clone of professor Dmitri Yuriev, URTV #669, along with Rubedo and Albedo, but they consider themselves brothers, Citrine then being their sister. He killed Yuriev fewq before the Miltian Accident, and had his life saved many time by Citrine. His body will latter one be used by Yuriev for his come back, until Albedo helps to defeat Yuriev once and for all. Of all, he is the URTV seeming the most bound to Citrine.