|Japanese VA:||Kazuyuki Okitsu|
|Character type:||Servant (Master: None)|
Caster (キャスター, Kyasutā?) is a Masterless Caster-class Servant granted with control over the security of the SE.RA.PH in the Moon Cell Holy Grail War. He is an independent Servant in the conflict of Fate/Extella.
He favors logic and rationalism, does not stray from his own views if possible, and endeavors to treat all things with objective dignity.
The Caster Servant Archimedes appears before Nero, who fights for the two Regalia. Archimedes is a standalone Servant summoned as a maintenance engineer for the Moon Cell Automaton, the spiritronic computer that created the spiritronic virtual world SE.RA.PH. He says that the 14,000-year refresh cycle for the Moon Cell's systems is happening right now. However, he declares that the Regalia, articles of authority, are necessary for the system refresh; they are incomplete, having been divided between Nero and Tamamo-no-Mae in an accident. He renews the determination of Nero and her Master to fight, not just for dominance over territory, but to stabilize the lunar world itself, which has begun to show signs of discord.
Caster cannot participate in the Holy Grail War due to his position as head of security, but he can still fight even if he cannot compete for the Holy Grail.
His Noble Phantasm is Katoptron Katho Phlegon ( Κατόπτρων Κατω φλέγον in Greek ).