Caster is the Servant of Soichiro Kuzuki and the Master of Assassin in Fate/stay Night. A talented magus, Caster's true identity is the Greek princess Medea, who was branded as a witch. During the fifth Holy Grail War, she kills her original master Atrum Galliasta and is eventually found by Kuzuki, who she falls in love with. Cold and calculating, Caster is willing to slaughter all of Fuyuki City to achieve the Holy Grail and win the war.
In the dub of the 2006 series, Caster was played by Tara Platt. For Caster's appearances in the ufotable adaptations, she was instead played by Megan Hollingshead.