Empress Xehanort is the true main antagonist of KH: ITHOTCR. She is the most depraved character of all Xehanorts and in Kingdom Hearts series. She is voiced by Grey Delisle-Griffith who voiced High Priestess from Samurai Jack. According to Villains fanon wikia, she is the only character in KH fanon to be purely evil, step aside The Coachman, Frollo, Lotso, Ardyn, Kefka, Mateus.
The final battle assures where Fulmen and Lumen's warriors are scattered so Chronus and Umbrae have to split up to bring the warriors and defeat the female embodiment of Xehanort.
She is nothing more than a pawn of Planet Xehanort and true evil soul of the Animus Umbri. She calls Master Xehanort a pawn and she is truly responsible for brainashing Master Xehanort. She is the mother of 13 seekers of Eternal Darkness.
In the past during a thousand year Keyblade war, she woke from vessel of darkness.