Caster (キャスター, Kyasutā?) is the Caster-class Servant of Kadoc Zemlupus in the Lostbelts of Fate/Grand Order. She is later able to be summoned by Ritsuka Fujimaru.

Caster's True Name is Anastasia Nikolaevna Romanova (アナスタシア・ニコラエヴナ・ロマノヴァ, Anasutashia Nikoraevuna Romanova?). The youngest daughter of Nikolay II, the last Tsar of the Russian Empire. Swallowed by the turbulence of the Russian Revolutions, she was murdered at the Ipatiev House along with her family, servants and pet animals.

Fundamentally naive and innocent, but due a misanthropic disposition, she speaks in a dispassionate tone right after being summoned - a stage in which she still does not have faith in the Master. If one were to earn her trust, he will probably see her original lively, mischievous side. She left behind many pleasant episodes, such as taking her own picture by projecting the camera with a mirror... something that modern people would call a "selfie".

Many of her letters to her family still remain, which gives us a glimpse of her innocent side and deep affection to her family. Strictly speaking, the spirit or monster called Viy does not exist. It is a fictional monster that appeared in the horror novella "Viy", written by the Russian literary master Gogol. However, there are several folklores similar to Viy in Slav culture, and one can surmise that it was modeled after them.

The Viy that Anastasia is contracted with was a familiar in the possession of the Russian Empire. By means of his mystic eyes, he exposed all secrets, discovered weak points in fortresses and also dyed enemies in blood. However, it is impossible to either see or control him without the necessary talent───. It was only in the brink of death that Anastasia attained the qualifications to make a contract with him.