Caster (Fate/stay night)

"If the Holy Grail has infinite wealth, it should not run out no matter how much we split it. Then can you not share the Holy Grail with those you trust?"

- Caster, Fate/stay night

Caster is a villainess from Fate/stay night who is the Servant of Souichirou Kuzuki during the fifth Holy Grail War. She also acts as the Master of fellow Servant Assassin.

Identity
Caster's identity is Medea, an unfortunate princess who was branded as a witch in Greek mythology. She was the princess of Colchis who possessed the Golden Fleece. Her father was King Aeëtes of Colchis who excelled at magic, and as his daughter, she was also practiced in such ways. Her personality at the time was far from what others would call a witch, and her fate became confounded after the famous hero from the Argo Expedition, Jason, appeared before her.

Jason argued with King Peleas, the usurper of his father's country Iolcos, and eventually made him promise to return the country upon traveling far eastward to Colchis and returning with the Golden Fleece. He was supported by the Goddess Aphrodite, and decided to control Medea's mind to help Jason upon realizing that the King of Colchis was unshakable. Medea was made to blindly love Jason, which resulted in choosing a foreigner over her father and country and therefore betraying them. Her abilities allowed her to nullify her father's magic and the enchanted fire-breathing oxygen, which gave Jason the opportunity to obtain the Golden Fleece.