Caster (Fate/stay night)

Caster is a servant from Fate/stay night.

An honored guest of the Ryūdou family, permitted to live at their temple until the preparations for her marriage have been finalized. A gorgeous and talented yet mysterious woman of high stature, her presence has attracted the attention of many trainee monks.

True Identity
The Eirei that is summoned as Caster was known as Medea during her lifetime. Under the tutelage of her father, King Aeëtes of Colchis, she became a skilled sorceress, but lived a quiet, peaceful life until the arrival of Jason and the Argonauts on their quest for the Golden Fleece.

Under the influence of god Eros, she falls madly in love with Jason and plays a key role in acquiring the Golden Fleece. She then leaves with Jason and bares him two children. Supporting her husband with her skills, she helps him in reclaiming his rightful title as the king of Iolcos, going as far as murdering his uncle who has usurped the throne.

After they were both exiled from Iolcos, Jason abandons her for the sake of strengthening his political bonds with marriage to Glauce, the daughter of Creon, the king of Corinth. Enraged and in pain, she gives Glauce cursed robes as a wedding gift. Upon wearing them, Glauce bursts into flames and her father dies refusing to stop embracing his only daughter. To complete her revenge to Jason, she kills their children and flees to Athens.

There she marries King Aegeus of Athens and lives a blissful and happy life, giving birth to a son she names Medus. After recognizing Theseus, the lost son of Aegeus in a newcomer to Athens, she attempts to kill him in order to ensure her son's inheritance to the throne. King Aegeus recognizes his son in the last moment and exiles Medea and Medus from Athens.

Having nowhere to go, they return to Colchis, Medea's homeland, only to find her father murdered and his brother Perses claiming the throne. In order to save his son, who was imprisoned by Perses to protect his position, Medea allows herself to be sacrificed to gods, only to assassinate her uncle with her last breath during the ceremony.

Her sacrifice allows Medus to claim the throne and in her honor he later renames a conquered nearby country.

Role
Typically, Heroic Spirits of an evil nature like Caster are not eligible for participation in the Fuyuki Holy Grail Wars; however, after the Holy Grail was tainted in the third war, most of the restrictions against such spirits were lifted. Caster's first Master was a member of the Mage's Association, but Caster killed him in retribution for his cruelty and abuse. On the verge of her spiritual body dissipating, she stumbled upon Kuzuki Souichirou at the foot of the Ryuudouji Temple. Caster begged this stranger for his help; Souichirou obliged without hesitation, forming a new contract with her. He then brought Caster into the temple and attended to her recovery. Caster quickly fell in love with Souichirou, for he was the first man to show her genuine kindness, dedication and loyalty. Her primary objective shifted from obtaining the Holy Grail for herself to simply preserving their short time together. To achieve this end, she has been siphoning Mana from the populace of Fuyuki City and has been fortifying her position on Mount Enzou.

Caster is killed in every scenario of Fate/stay night. In Fate, her stronghold in the Ryūdouji Temple is briefly assaulted by Saber, though her own Servant, Assassin, manages to hold her back. She is later torn apart by Gilgamesh's Gate of Babylon while ambushing Emiya Shirou and Saber. In Unlimited Blade Works, she manages to stab Saber with Rule Breaker and negate her contract with Shirou; Caster then abducts Saber and tortures her, hoping to provoke Saber into becoming her Servant. While attempting to kill Emiya and Tohsaka Rin in the basement of the Kotomine Church, she dies protecting her Master from Archer's hailstorm of blades. Caster is killed early on in Heaven's Feel by Saber, but Matou Zouken uses one of his worm familiars to maintain her body as a puppet.

In Fate/hollow ataraxia, Caster goes about as a normal woman, normally hanging around the mall. She is the first person to discover the four-day time loop (other than the player). She also fights the dog-shadows using anti-army spells, often coming close to killing her allies.