The controlling (but caring) oversoul. Thought to be tied by philotic connections to the body, the aiu is the total of a person, their essence. Derived of an unknown substance, aiu very in size and power, seeking control of a physical body in our universe.
All bodies (celestial or earthly) have an aiu. Commonly refered to as an oversoul, the aiu's power and size directly corrolate to the strength of their physical body's personality.
(see philote, Orson Scott Card, or soul)