She is another personality that resides within Aris's mind, and represents her original programming to carry out a genocidal campaign in the name of the "Nameless Priests" who created her. She often refers to Aris as "Princess" in reference to this role. She was originally referred to as "<Key>", as the program was meant to unlock Aris's 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 is revived and her consciousness is loaded onto a purpose-built AMAS robot produced by Rio.

In the chapter "Sky of Unity" and the 6th PV, Kei manages to obtain a body from a discarded unit from netzach and she is now capable of using her Nameless Guardian and Decagrammaton powers.