The core system of The World; introduced in .hack//Sign as an omnipresent AI that was created to help Aura develop.Eventually she realizes that once Aura is born, she will lose her purpose.Thus, Morganna links Tsukasa to Aura, which put Tsukasa's real life self into a coma, and attempts to push him into utter despair to feed only negative thoughts to Aura in order to stop her development.

(Source: Wikipedia)