Caster (キャスター, Kyasutā) is the Caster-class Servant of Ryuunosuke Uryuu in the Fourth Holy Grail War of Fate/Zero. He is one of the Servants of the Protagonist of the Grand Order conflicts of Fate/Grand Order.
Commonly known as Bluebeard, his real name is Gilles de Rais - a mass serial killer who seems to favour killing children in particular. Caster is a fiendish Servant who derives pleasure from first providing his victims a moment of relief before terrorizing them and then killing them. He expresses an obsessive interest in Saber because he has mistaken her for Jeanne d'Arc, his former comrade and the protagonist of Fate/Apocrypha, with whom Bluebeard has an infatuation. As his wish for the Grail was for her to be resurrected, Caster becomes convinced he has won the War and repeatedly tries to talk to Saber; when she subsequently tries correcting him, he believes that she has lost her memories by God's intervention, much to his rage. He has, by far, the best relationship with his master of any servant. He sees Ryūnosuke as a prophet who preaches that God is a sadistic yet loving being that enjoys seeing His creations commit horrific acts, as He wouldn't have made the human race so violent and their organs so colorful if He did not enjoy chaos. This restores Caster's faith in God and he decides to put on a show for Him, to show Him that He isn't the only "entertainer" who can put on a show. The war is eventually postponed due to the duo's rampage, as Caster and Ryūnosuke had begun to murder hordes of children in hopes of catching God's attention and forcing Him to "return" Saber's non-existent memories of being Jeanne. Following Ryūnosuke's death, Caster is obliterated by Saber's Excalibur.
Caster's sole Noble Phantasm is Prelati's Spellbook: Text of the Sunken Spiraled City (螺湮城教本（プレラーティーズ・スペルブック） Purerātīzu Superubukku), a magical tome made from human skin which acts as a massive mana source and a guidebook for various spells of varying power, with the strongest spell capable of summoning otherworldly, self-regenerating monsters. The strength of his Noble Phantasm helps make up for his otherwise average ability in magecraft.