Maxwell's Demon (マックスウェルの悪魔, Makkusuweru no Akuma?), Class Name Caster (キャスター, Kyasutā?), is the Caster-class Servant of the Fuhrer in the Imperial Capital Holy Grail Strange Story of Fate/KOHA-ACE. He is the Servant of High Priest Makuzu in Fate/Grand Order.
Maxwell's Demon is a fictional demon created by James Clerk Maxwell as a thought experiment, distorted by the desires of people.
Caster has short, well groomed orange hair, and is dressed in a tan business suit with a red tie and sunglasses. While his general appearance is unchanged in his appearance in Fate/Grand Order, his colour palette has shifted drastically to light brown hair, a two toned green suit and black tie.
GUDAGUDA Final Honnōji
Caster came to check on Assassin after his conversation his Master. Assassin thought Caster came to get his report from him, he reminds Caster about his Master's report on him, he did got the report. When Assassin thinks he refers to the magus he fought, but he wasn't talking her, he is talking Saber's Master. After Assassin informs him what he knows from the other day and asks why is he's interesting in him, he smile and asks Assassin he would be appreciate if he tells him more about him.
Rather than being a magus, he is a scientist Heroic Spirit. Due to the nature of Maxwell's Demon not being resolved as of the 1940s, Caster is impossible to kill, except by attacks that kill even unkillable existences. He possesses no Class Skills or Stats in anything other than Noble Phantasm, and his only Skill is Devil's Proof, however no description of the skill is given. His Noble Phantasm is also called Maxwell's Demon.
In normal circumstances, he cannot be summoned at all. Only when humanity discovers the secret to generating unlimited energy will he be able to answer a summons without serious consequences.