The Testaments are Wilhelm's closest assistants and servants. Characterized by colored robes with matching masks, the true nature and power of the Testaments is unknown. It is hinted that they possess the power to manipulate space, and they seem to be able to appear and disappear at will. Also, each Testament (as well as Wilhelm himself) appears to have their own E.S., a special kind of spacecraft that contains a Vessel of Anima. However, this has not been confirmed, and the E.S. could merely be powerful A.M.W.S. mechs, or an entirely new breed of gears. As of the end of Xenosaga Episode II, four Testaments have been revealed: red, blue, black, and white.
In terms of allusions and references, the four Evangelists John, Mark, Matthew and Luke who wrote the Gospels which are part of the Testament are sometimes depicted as wearing cloaks of these four colors. The four compass directions in several cultures, including Feng Shui and Aztec mythology, are equivalent to these four colors.
Wilhelm hints that the role of the Testaments involves "Influencing Space-Time Anomalies"; that much becomes clear when they resurrect Albedo and help to create a space-time anomaly which engulfs the Omega System. This may be part of a much larger plan that is generally believed to involve recovering or returning to Lost Jerusalem.
The Testaments are incredibly powerful and may easily rank as the most powerful entities in the series. None of them appear corporeal and are more spirit. They apparently can also transform into larger Gnosis monsters which may explain their ability to teleport (chaos states at the beginning of Episode II that no human can teleport or enter hyperspace safely without significant protection). They can also seemingly subvert the laws of physics to perform phenomenal feats as the Blue Testament states: "The laws which govern us are just too different" after he easily brushes aside the party. In order to become a Testament, the person the Testament was must die.
Main Article : Voyager
The first and oldest of the four current Testaments. He appears for the first time in Xenosaga: Pied Piper as a U.M.N. cyberterrorist named Erich Weber who also went by the hacker alias Voyager. He was responsible for the death of Jan Sauer's (a.k.a. Ziggurat 8) wife and stepson. Of all four Testaments, we see him in action the most, though he doesn't appear much until the second game. In neither game has he spoken a word. He has very pale skin, no hair, and bright red eyes. He pilots a black humanoid E.S. with large, plate-like wings and fanned tail, accompanied by numerous cannon drones. When in transit to Old Miltia, Shion and Allen were attacked by him, whether this was to stop them or possibly gauge KOS-MOS's abilities when she promptly rescued them, is currently unknown.
In Xenosaga I&II his role is significantly expanded and he actually gets speaking lines. He speaks to Ziggy on the Omega System after unmasking for the firs time. This leads to Ziggy opening up about his past to Shion and the others later in the game. Before that he confronts U-TIC / ORMUS AMWS pilots Richard and Hermann and speaks with them. In Xenosaga III, Voyager confronts the party the most and actually speaks to Ziggy after a battle.
Main Article : Kevin Winnicot
The next oldest Testament (presumably). His true identity that of Kevin Winnicot, Shion Uzuki's lover who was suposedly killed by the Archetype KOS-MOS. The Red Testament appears to be Wilhelm's right hand man and is described in some leaked production to be the leader of the Wilhelm's "Vanguard Forces" the Testaments. The two of them have extended intimate conversations in several scenes throughout both games; in one scene they even play chess together. When the Testaments appear on the Omega System near the end of Episode II, Shion recognizes Kevin's voice when he addresses the Patriarch. Some have suggested that the character of Roth Mantel, the chief Vector scientist working on the T-elos project, is actually the Red Testament operating in disguise. Interestingly, "Roth Mantel" is "Red Cloak" in German. He pilots an agile, red E.S. with blade-like horns, similar to the Xenogears antagonist Grahf.
Main Article : Luis Virgil
This Testament was created during the course of the Xenosaga Episode I. He is known to be Luis Virgil, a man who was killed by KOS-MOS during the Gnosis attack on the Woglinde. He appeared to have a violent hatred for Realians, and suffered from the potent DME Addiction. Even as a Testament his face still shows the cracking pattern characteristic of people with this addiction. In a mysterious comment in Xenosaga: Episode I, he refers to chaos as boss. He first appears as the Blue Testament on the Song of Nephilim where he bails out Albedo who was just defeated by Shion, Jr. and their companions. Albedo departs, telling the Blue Testament to give 'him' (most likely Wilhelm) a message: "'it' (most likely either Proto Merkabah, MOMO, or U-DO) belongs to me (Albedo himself). When Albedo departs Virgil attacks the group using his new found powers as a Testament, which include but are not limited to; subverting the laws of physics and transforming into a Gnosis. Moreover, the type of Gnosis that the blue testament transformed into (Ein Rugel) was also the same type of Gnosis that the Black Testament Voyager turned into when Ziggy aka Jan Sauer confronted him over 100 years ago. Testament appears again after Albedo summons Proto Merkabah to Second Miltia and he is reprimanded by the Red Testament to just let Albedo be for the while. When the Red Testament shows him just what his and Wilhelm's plan is and how it relates to Albedo, he reluctantly backs off. He pilots E.S. Napthali, a blue, arachnid-limbed machine with a humanoid torso and a large accompanying rifle that can be teleported at will.
In Xenosaga I&II Virgil and E.S. Naphtali show up on Old Miltia in the Episode II portion of the game and he confronts Shion and her companions before they reach Labrynthos. He then attacks the group and they do battle. He later speaks with Shion before she faces Patriarch Sergius on the Omega System. In Episode III, he appears fequently attacking and confusing the party. Once he attacks in her E.S. and again as himself with his Gnosis-like guardian assisting him in battle.
Main Article : Albedo Piazzolla
The newest Testament, who appears at the very end of Xenosaga Episode II. While his identity is a mystery at the end of Episode II, in Episode III it is revealed that the White Testament is in fact U.R.T.V. Albedo Piazzola, who had recently died in a confrontation with his brother Rubedo (a.k.a. Jr.). Wilhelm refers to him as the "weaver of the eternal circle of Zarathustra". He pilots a white E.S. with a rectangular body and suspended blades instead of limbs, with the "head" portion raised like a church steeple.