Experimental anti-Gnosis android from the Xenosaga franchise.
KOS-MOS has significant powers believed to be superior to all other party members. In addition to superhuman strength and speed, her body is formed from some sort of highly advanced nanomachines that allows her to morph parts of her body (specifically her arms) into various energy projectile and melee weapons. Her body is also able to self-repair and is very durable though not indestructible, and she is also protected by a force-shield. She also possesses the ability to summon weapons such as the F-GSHOT chainguns and her famous F-SCYTHE and equipment from another location via U.M.N. transport. She also has a long-range sensory array, the D.S.S.S. system and CPU that exceeds even those possessed by Observational Realians.
She also possesses a Hilbert Effect projector whose power is thousands of times greater than that achieved by even the largest spaceships.
KOS-MOS possesses several black box components left behind by Kevin Winnicot. Each of these black box features are found in each of her skeletons, version 1, version 2, version 3, and version 4. One of these is her potent X-BUSTER weapon, a powerful burst of energy beams from her abdomen that absorb the Gnosis. Others are related to her "simulated personality" OS and her self-awareness which is more in fact Mary Magdalene resting inside of her. KOS-MOS also has some form of energy manipulation tied to the Zohar. The Red Testament, Kevin Winnicot, claimed that KOS-MOS's power came from "U-DO using the Zohar as a door".
KOS-MOS has another weapon system called the Tertiary Weapon System, which is seen periodically throughout the series. They consist of a set of energy wings, a pair of energy cannons and other attachments meant to be fitted onto her. The wings and cannons are part of a Phase Transference Cannon. While wearing this gear she is shown in a premonition flying through space at sub-light speed and then confronting U-DO above an unknown planet. The exchange that follows destroys several galaxies. The "Third Armament" as its also known makes a real-time appearance in Episode II on the Ω System. KOS-MOS uses this equipment to protect Shion and the party from U-DO. In Episode 3, KOS-MOS uses the tertiary weapon system to penetrate the barrier that surrounded Rennes-Les-Chateau in order to help the Elsa crew. The Tertiary Weapon System is really only compatible with her Version 2 and Version 3 skeletons.
While she is programmed to protect Vector personnel unconditionally, her highest priority is to protect Shion specifically. As seen early in Episode I, she has the capacity to ignore Shion's orders if Kevin Winnicot (the Red Testament)'s pre-programmed directives supersede them. In Episode III, shortly after KOS-MOS is destroyed by T-elos and subsequently rebuilt she rejects the direct commands of Kevin Winnicot. She is rebuilt with some of the Professor's Erde Kaiser technology, which makes her much more powerful than before.
An almost human alternate persona tends to come out from time to time in the form of a blue-eyed KOS-MOS. This blue-eyed KOS-MOS is far more powerful than red-eyed KOS-MOS and talks in a more empathetic human-sounding voice. This alternate KOS-MOS is in fact KOS-MOS' true personality that partially awakens at times when Shion is in extreme danger. Her true identity is that of Mary Magdelane.
Mary makes several appearances in Xenosaga: wiping out a fleet of Gnosis surrounding the Cathedral Ship with her X-Buster attack, asking chaos to save the Elsa from burning up in the Second Militian atmosphere using his Anima power while providing 'cover' by seeming to protect the ship herself, and saving Shion and Allen from being killed by Voyager, the Black Testament, allowing the party's E.S.s to enter Rennes-le-Chateâu and at the end of Episode III.
Mary also asked Shion during her first appearance in Episode I, "Will feeling pain make me complete?", though Shion could not give her an answer because she could not hear her voice in the vacuum of space. Shion at first had not directly seen KOS-MOS as Mary and was oblivious to her existence until the later chapters of Episode III.
KOS-MOS was created by Kevin Winnicot to hold the soul of Mary Magdalene, who possesses the power of Animus. According to the Official Complete Guide, Mary was the human incarnation of Animus, a power that complements Anima. While KOS-MOS has Mary's soul, T-elos was built using Mary's body. In order for Mary to fully awaken, one must be absorbed by the other. Long ago, Mary had died in the act of "sealing' 'chaos' Anima power, separating it from him and dividing it into the twelve Anima Vessels. This also resulted in Mary's will being scattered across the U.M.N. It is stated that Shion, being the incarnation of the "Maiden of Mary", is needed in order to retrieve Mary's soul from the U.M.N.
Mary awakens in KOS-MOS instead of T-elos, which results in the transformation of Mary's original memories and characteristics into an entirely new personality which is both Mary and not Mary at the same time. The game's database compares it to Sakura's transformation into MOMO, Wilhelm planned to use KOS-MOS' control of the Anima through her emotional bond with chaos to start Zarathustra. However, she rejects Wilhelm's plan and destroys the "Key" (Shion's pendant), saving Shion from Wilhelm's torture.
After the defeat of Zarathustra, KOS-MOS stays behind with Nephilim, chaos, and Abel in order to assist in Nephilim's plan to send the collective unconsciousness of humanity to Lost Jerusalem. She parts ways with Shion in an emotional goodbye, promising that she will see her again; Shion tells her that it was nice to finally meet the "real you". KOS-MOS then transfers her Animus power into Nephilim (according to the database, this is the Animus power, not Mary's will), to enable her to carry the fractured wills (gnosis) that chaos guides into her with his power. She protects Nephilim and chaos from the invading Gnosis as they focus on completing the dimensional shift to Lost Jerusalem.
In the last moments of the battle, KOS-MOS sacrifices herself to destroy a Carrier-type Gnosis (which resembles an enormous blue-colored sperm whale). According to the database, her last words referred to how she was unable to save Shion's past incarnation from dying in ancient times, but now in this lifetime she could save her. The battle ends with KOS-MOS being heavily damaged, losing an arm and both legs and being left behind after Abel's Ark, with Michtam inside it, is pulled into hyperspace. She then begins a long journey through space, chaos' spirit comforting her, and telling her to sleep until the time they are needed again.
She is last seen silently floating towards a single blue planet with white rings before a rising sun. This planet is the restored Lost Jerusalem—Earth. This is the last scene of Episode III