In his past life, Caster was Gilles de Rais, a French nobleman who was once a comrade of Joan of Arc. He greatly admired the woman when she was alive and her death devastated Gilles to the point where it completely shook up his faith in God. He studied black magic under Francois Prelati and committed a number of atrocious crimes with his magic for eight years until he was captured and executed.
Gilles' name of Bluebeard that he adopts for the Holy Grail War stems from a fictional mass killer of children based on his character.
Caster stands at 6 feet, 5 inches and is normally adorned in long black robes with different designs adorning the top and bottom of his robes.
Personality Like his Master Ryuunosuke, Caster is psychologically demented and has no regard for any of the lives he takes. He occasionally likes to set up situations to allow his victims a false sense of hope by letting them escape before brutally killing them. Throughout the Grail War, he is under the belief that Saber is his former comrade, Joan of Arc, due to their physical resemblance to one another.
Caster was accidentally summoned into the 4th Holy Grail War by Ryuunosuke using the blood of a family that the latter had killed. Because Ryuunosuke lacked a catalyst to summon Caster, the Grail chose a Servant for him with the closest personality they shared. Rather than take active involvement in the Grail War though, Caster and Ryuunosuke would abduct a number of children throughout Fuyuki and use them either as mediums for summons through Caster's spellbook or torture them for their sadistic pleasures. This lack of regard for the rules of the Holy Grail War led the war's moderator, Risei Kotomine, to temporarily suspend activity in the Grail War and offer the reward of extra Command Spells for any Master/ Servant pairing that killed Caster.
Caster would use the children as hostages to lure Saber into a trap where she would be attacked by the countless summons created through his spellbook. Lancer would come to her aid and use Gae Dearg to disable the summons channeled through the spellbook and force Caster to retreat.
Caster would try to go after Saber a second time when he fused the creatures in his spellbook and himself into a giant, massive beast capable of constantly regenerating itself and having impressive strength. He would eventually be defeated when completely incinerated from an attack by Saber's Excalibur as he recalled the events that led to his murder spree before fading away.
Bluebeard's status as a Caster class stems from the use of his only Noble Phantasm, Prelati's Spellbook, as he otherwise possesses little ability in magic, making himself one of the weakest Casters on record in the Holy Grail War. Made of wet human flesh, the spellbook is capable of summoning countless demonic familiars with little use of Caster's mana reserves since he uses the spellbook as a medium for the summons. The endless summons of these creatures are what make Bluebeard a threat to others and disabling the mana output used in maintaining the form of the familiars through the spellbook is the only means of stopping their summons. Bluebeard is also capable of using the spellbook to teleport himself if he needs to flee from battle.
Bluebeard is also capable of fusing with the spellbook and the creatures within it to form a giant beast that can feel no pain and is capable of constantly regenerating itself from any injury as most attacks from Servants such as Saber and even Gilgamesh were incapable of incapacitating the creature. It also possessed a large amount of strength as it could casually swat Berserker's F-15 fighter jet out of the air with its tentacles and swallowing it. The only means of destroying the creature were to either kill Bluebeard's Master where it would eventually fade away from using up all the mana in its body (so long as it didn't absorb the life force of living beings to maintain itself) or using an attack destructive enough to completely incinerate the creature's form, such as Excalibur.