MedeaWP (メデイアWP, Medeia?), Class Name Caster (キャスター, Kyasutā?), is the Caster-class Servant of Souichirou Kuzuki in the Fifth Holy Grail War of Fate/stay night. She also acts as the Master of another Servant, Assassin.

Cruel and cold-hearted, she is a genuine wicked woman who does not choose the means to achieve her objectives and makes evil designs her forte. However, such personality was established as a reaction of the fate imposed on her. A tragical woman who, a result of constantly facing betrayal, ended up falling on the standpoint of betraying people herself.