"Nigredo is the chairman of the Kukai Foundation under the alias of Gaignun Kukai, a special organization based in Miltia, Nigredo is a genetically altered clone of professor Dmitri Yuriev, along with Rubedo and Albedo, but they consider themselves brothers. However, due to the fact that cloning is now considered taboo, Jr. charades as Gaignun's legally adopted son. He is the perfect leader for the massive and wealthy Kukai Foundation. Gaignun has black hair and deep green eyes. He has a gentle personality and is well respected both within and outside the Foundation. Gaignun is listed as the first son and heir to the industrialist Soze Kukai.

Gaignun Kukai is really U.R.T.V. Unit #669, who is a biological weapon designed to combat U-DO. U.R.T.V. Unit #669 is nicknamed Nigredo, a reference to his black hair. Gaignun can telepathically communicate with Jr. and Albedo and can use his voice to hypnotically manipulate people, if need be he can telekinetically destroy a person. "Kukai" was a fictional persona created by the Second Miltian government as a means to pool their special operation funds. Following the Miltian Conflict and Helmer's rescue of Gaignun and Jr., they took on the Kukai surname to justify and establish the Kukai Foundation. This also allowed the U.R.T.V. units to live their lives more normally.

In the series, Jr. and Gaignun both raised a cat named Gaignun, which is the origin of Nigredo's alias. "

-Taken from xenosaga.wikia.com