KOS-MOS is an android from the Xenosaga series. In Soul Calibur III, her appearance can be unlocked as the Type X Character Creation parts, making her a "pseudo-guest" character of sorts - no promotional material regarding her "inclusion" exists, and she possesses no official story in the series. As she is an android, a logical explanation for this would be that, due to her malleable abilities in her original series, she was included to highlight Soulcalibur III's new Character Creation system.