Caster served as Souichirou Kuzuki's Servant during the fifth Holy Grail War. She was initially summoned by a man who refused to take an active role in the Holy Grail War. Caster tricked him into using up his Command Spells before she found Kuzuki. Caster is a manipulative schemer who excels at magic and is skilled at reading the momentum of the war.
As a Caster class, Medea is exceptionally powerful at spell-casting. She is also adept at creating a territory where she turned the grounds of Ryudou Temple into a place that further empowers her spell-casting. Because of her vulnerability in direct combat, she resorts to any possible means to win the war, including summoning a Servant, Assassin. Caster's greatest weakness is Berserker, and her strategy to win the war revolved around defeating him.
Medea is the princess of Colchis who owned the Golden Fleece that Jason of the Argo Expedition wanted. The King of Colchis refused to give Jason the Golden Fleece, thus Aphrodite made Medea fall blindly in love with Jason. Medea helped Jason obtain the Golden Fleece while under Aphrodite's control.
The King of Colchis then led his army to capture Jason, but Aphrodite made Medea board Argo and cut her brother Apsyrtus to pieces right before her father's eyes. Upon returning to Iolcus, King Pelias refused to return the country to Jason and plotted to assassinate him. Jason relied on Medea's magic 3 times to escape the assassination attempts.
Medea then tricked King Pelias into getting killed and Jason reclaimed the throne. The Argonauts disapproved of Jason's methods and Medea's actions. She was denounced as a witch and was cast out of the country with Jason. They ran to Corinth and were welcomed by the king.
The King of Corinth took a liking to Jason and offered him his daughter's hand in marriage and Medea was exiled. Medea later attended Jason's wedding and burned everything and everyone there except Jason. After that, the common folk treated her as a witch and blamed her for all the misfortunes that they experience. Medea was powerless to change their views and instead took to hating everyone.
Caster was initially summoned by a passive man who wished to wait the war out. Tired of waiting, she tricked him into wasting his Command Spells and killed him. Caster then wandered around Fuyuki City cursing her fate when she found Souichirou Kuzuki. He helped her without question and took Caster as her Servant. She fell in love with him because he was the first sincere man to help her.
Since then, Caster used Ryudou Temple as her base of operations. She casted a spell on the city to siphon life energy from the local populace to fuel her magic. She also conjured a large barrier which surrounded the temple grounds. Any enemy Servant who attempt to force through will have their power drained. The only spot that is open is the front gate which is guarded by Assassin.
At the temple, Caster presented herself as Kuzuki's bride-to-be and she was allowed to stay at the temple in the guise of planning for their marriage.
Ryudou Temple was assaulted by Saber but she was denied entry by Assassin. Later, Caster attacked Shirou's home but she was killed by Gilgamesh.
Unlimited Blade Works
Caster took control of Shirou and led him to Ryudou Temple in order to steal his Command Spells and gain command of Saber. Saber came to his rescue but was caught in a duel with Assassin. Archer also showed up and Caster explained that she wanted Saber to defeat Berserker. Archer dueled against Caster which allowed Shirou to escape.
When Shirou, Rin, and Saber returned from their date, they found Caster holding Taiga Fujimura hostage. Caster used her Noble Phantasm, Rule Breaker on Saber and took command of her. She then assaulted the church and ousted Kotomine where she subjected Saber to her Command Spell in attempt to bend Saber to her will. Archer then betrayed Rin and accepted Rule Breaker as well.
Later still, Shirou, Rin, and Lancer assaulted the church. Archer then betrayed Caster in turn and attempted to kill Kuzuki with a shower of swords. Caster took the attack in his place and confessed her love for him in her dying breath.
Caster was killed by Saber after Ryudou Temple was assaulted by the black shadow. After Shirou and Saber left, Caster was devoured by the black shadow. The black shadow then regurgitated Caster for Zouken and he animated her using his worm familiars and used her to attack Rin, Archer, Shirou, and Saber in town. Caster was struck down by Saber again.
High Speed Divine Language
Caster's ability to cast extremely powerful spells near instantly.
Caster boasts an impressive array of spells at her disposal. Among them is Strengthening, which she used to strengthen Kuzuki to the point of being able to match Saber in direct combat.
Caster is able to conjure images of herself to fool her opponents.
Caster is able to swiftly teleport to a chosen location. She teleported to Kuzuki's defense on the Unlimited Blade Works route.
Caster is able to take flight. She attacks from up high where she cannot be reached with melee attacks. She used it once against Archer on the Unlimited Blade Works route.
Caster can summon skeletons animated by magic to attack and swarm her enemies. The skeletons are skilled at melee and they are usually found wielding a rough sword.
Saber could defeat them easily but Caster uses their sheer number to overwhelm her. Caster used them once to attack Shirou's home on the Fate route.
Rule Breaker is a dagger with a strangely shaped blade. Its blade is very weak and is barely able to stab a human.
Its ability is to nullify magic effects. Caster uses it to break the contract between a Master and his or her Servant and take the Servant for herself.
She threatened to use it on Saber on the Fate route, and used it on Saber and Archer on the Unlimited Blade Works route.