Gilles de Rais is a Servant in Fate/Zero, summoned as Caster. He is a twisted, fanatical mage and former noble inspired by the historical figure of the same name. In the series, he serves as one of the most unsettling antagonists, obsessed with Joan of Arc and driven by madness, cruelty, and grotesque devotion. His role is to amplify the horror elements of the Holy Grail War, acting as a dangerous and unstable threat to both civilians and other Masters and Servants.