She is another personality that resides within Aris' mind, and is the representation of her original programming to carry out a genocidal campaign in the name of the "Nameless Priests" who created her. She will often refer to Aris as "Princess" in reference to this role. Originally she was referred to as <Key> as the program meant to unlock Aris' nature as the Princess of the Nameless Priests, but would be given the name of "Kei" during the events of Volume F as Aris accepted her as a part of her existence.

In Decagrammaton Chapter 2 "Sword of Flames" Kei has been revived and her consciousness was loaded onto a purpose-built AMAS robot produced by Rio.