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See also Lucifer (alternate)
Lucifer is the Cylon IL-series attached to Baltar, who is assigned to seek out Galactica's Fleet and destroy them. He initially presumes that he will learn much from the Human who betrayed his own race (Saga of a Star World).
Baltar and Lucifer demonstrate from the beginning of their relationship a drastically different way of thinking. Lucifer appears to have a very good grasp of strategy and tactics, yet he is unable anticipate Baltar's orders. Lucifer at first views this unpredictability as an asset, and is eager to observe Baltar. Lucifer is curious to see how Baltar reacts when Galactica ends up behaving in a way that neither of them seem to have anticipated, and is delighted to study under so devious a specimen (Lost Planet of the Gods, Part I).
Lucifer grows impatient with Baltar's games very quickly, and seizes command of the basestar when Baltar goes to Kobol to try to lure the Colonials with promises of peace. He orders an attack on the Colonial forces, inadvertently freeing Adama, Apollo, and Serina as well as trapping Baltar beneath a fallen stone inside the Tomb of the Ninth Lord of Kobol (Lost Planet of the Gods, Part II).
His final appearance is in part 1 of War of the Gods when Baltar decides to seek out the Galactica. Lucifer questions that the humans would accept Baltar's offer of alliance, yet he doesn't attempt to prevent Baltar from going on his foolish expedition, indicating that he isn't all that disappointed to have Baltar gone.