KOS-MOS is an armored gynoid, developed by the interstellar conglomerate Vector Industries. She is comprised of entirely mechanical parts (possibly from a superior breed of complex nanomachines), a rarity in the age of advanced Realian technology. The basic tenet to which she adheres is based on logic, probability, and completion of her mission above all else. She also unconditionally protects Shion Uzuki, her co-creator. She is equipped with a "Simulated Personality OS" to aid in communication, and though her speech conveys her role as an gynoid in service to humans, her tendency to put logic and probability first makes her quite difficult to manage at times.
The name 'KOS-MOS' is a recursive acronym used to denote an anti-Gnosis tactic system. It stands for "Kosmos Obey Strategical Multiple Operation Systems". Her development name is "KP-X", and she is self classified as a "Special Humanoid Anti-Gnosis Annihilation Weapon". Her serial number is "00-00-00-00-1". She appears to be emotionless, but throughout the series, evidence indicates that this is not entirely true.
An almost human alternate persona tends to come out from time to time in the form of a blue-eyed KOS-MOS. Blue-eyed KOS-MOS appears to be more powerful than red-eyed KOS-MOS and talks in a more human-sounding voice. This form of KOS-MOS only seems to appear at times when Shion is in extreme danger. Therefore, this version of KOS-MOS plays the role of Shion's guardian.
Blue-eyed KOS-MOS has made three appearances in Xenosaga so far: wiping out a fleet of Gnosis surrounding the Cathedral Ship with her X-Buster attack, saving the Elsa from burning up in the Second Miltian atmosphere, and saving Shion and Allen from being killed by Voyager, the Black Testament. Blue-eyed KOS-MOS also asked Shion during her first appearance in Episode I, "Will feeling pain make me complete?," though Shion couldn't give her an answer because she could not hear KOS-MOS' voice in the vacuum of space. Shion has not directly seen KOS-MOS in her blue-eyed form and is oblivious to this second persona's existence until Episode III.