Lucifer of the Frozen Lake of Cocytus is a major antagonist in the Kingdom Hearts Unlimited Saga, first appearing in "The Keyblade Wars" as one of the co-conspirators behind the Old One alliance attacking the Realm of Light. He later appears in "Return of the Keyblade" in a shadowy avatar of his normal appearance due to being sealed in the deepest abyss of the Realm of Darkness, before finally being released near the climax of the story to rejoin his brothers.
One of the more powerful of the original Great Evil's shards that were split apart during the Age of Chaos, Lucifer represents the being's insurmountable Pride in himself and his raging Envy towards all others of his kind who were able to be freed before him and against TOWIAA for locking him in the Cocytus River just because he refused to bow down to his authority. Because of his sheer power that he could unleash if he were freed, he is currently being sought after by both Maleficent of the Hellfire Organization and Mok Swagger of the still fledgling Nightbreed Organization in order to recruit him to either of their alliances. Instead, he declines their offers and sides with his brothers-in-arms because he feels he's much too important to be seen with such mortal trash who are so beneath his level it's not even funny.