|Appears in:||Fate/Prototype: Fragments of Blue and Silver|
|Character type:||Servant (Master: Manaka Sajyou)|
Caster (キャスター, ?) is the Caster-class Servant of Misaya Reiroukan's father in the First Holy Grail War of Fate/Prototype: Fragments of Blue and Silver. He later forms a new contract with Manaka Sajyou.
Caster is an alchemist from the Renaissance period and the one responsible both for creating the Alchemic system that is used by most alchemists of the modern era as well as creating the Azoth sword, which has become a symbol of a right of passage for Mages, with copies of the original being given by Mages to their students as a simple, yet very effective Mystic Code.
Given his nature as an alchemist, he is able to create a large number of homunculi to do his bidding in a short time span, with each of them being connected to each other as a hive mind. In addition to his artificial humans, he also possesses a large number of familiars.
Being a member of the Caster class, he is able to set up several types of Bounded Fields and magical traps, effectively turning his stronghold into a forest of death.
Contrary to his quest of progressing life and helping others, he is also remarkably skilled in curses, being able to stealthily inscribe a lethal curse upon a magus without them noticing.
Paracelsus is also an Average One, meaning he can manipulate all of the five Elements. He is very adept at jewel magecraft, effectively having stones packed with prana floating around him in combat and throwing them against targets in a way similar to telekinesis. They are powerful enough to blow away the "shadows" thrown at him by Manaka, a human who is directly connected to the Root.
One other field in which he seems to have a good deal of experience in is Healing Magecraft, as he offered his services to Saber, but there was no need as Manaka had already seen to his recuperation.