Caster's True Name is Maxwell's Demon (マックスウェルの悪魔, Makkusuweru no Akuma), 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.
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.