The Servant known as Caster from Fate/stay night is a tragic figure whose true identity is Medea, the princess of Colchis from Greek mythology. She was the daughter of King Aeetes, a powerful magus, and a niece of the sorceress Circe, which placed her in a lineage of exceptional magical ability. In her lifetime, Medea was a quiet and peaceful princess until the hero Jason arrived on a quest for the Golden Fleece. Driven to madness by the goddess Aphrodite to fall blindly in love with Jason, she betrayed her father, helped Jason steal the treasure, and killed her own brother to aid their escape. After Jason abandoned her to marry another woman, Medea took a terrible revenge, killing his new bride and, in many versions of the legend, her own children, an act that cemented her reputation as a murderous witch. This history of betrayal and being used as a tool by others defines her character.

Personality wise, Caster is a complex being shaped by immense suffering. For most of her life, she was treated unreasonably, never understood, and cast as a convenient scapegoat for the ugliness of the world. This led her to accept the role of a witch, vowing to show people the darkness they demanded to see. While she is a cunning and ruthless strategist in the Holy Grail War, merciless to her enemies and capable of draining the life force from innocent citizens to fuel her magic, this is a hardened exterior. Her true nature is that of a pure-hearted, romantic princess who dreams of a peaceful life. She dislikes being called a witch and has a love for adorable clothing, though she feels it does not suit her, so she instead appreciates seeing it on others. She also has a noted dislike for muscular men and a preference for quiet, sincere individuals.

Caster's primary motivation drastically changes over the course of the story. Initially, like all Servants, she desired the Holy Grail to grant a wish, though her wish was likely born from her desire for revenge against a world that wronged her. However, her entire outlook transforms upon meeting Souichirou Kuzuki. Her original Master was a conventional magus from the Mages Association who treated her with suspicion and disdain, attempting to limit her power. After she killed him to escape his control, she was on the verge of disappearing when the stoic and emotionless teacher, Kuzuki, found her. For the first time in her life, a man showed her genuine, unconditional kindness and loyalty without any magical coercion or ulterior motive. Her primary objective shifts instantly from obtaining the Grail for herself to preserving her fleeting happiness with Kuzuki, her new Master and the man she comes to love deeply.

In the narrative of the fifth Holy Grail War, Caster plays the role of a formidable and cunning secondary antagonist. She establishes her base of operations at the Ryuudouji Temple, a powerful spiritual land, using her Territory Creation skill to construct a temple-class workshop that drains mana from the surrounding city. She exploits a loophole in the Holy Grail War system to summon her own Servant, the wraith-like Assassin (the swordsman Sasaki Kojirou), to guard the temple gate. Her most significant act is using her Noble Phantasm, Rule Breaker, to sever Saber's contract with her Master, Shirou Emiya, and forcibly claim the powerful knight as her own Servant.

Her most critical relationship is with her Master, Souichirou Kuzuki. Unlike a normal Master-Servant pair bound by a magical contract and Command Spells, their bond is one of mutual salvation. Kuzuki, a former assassin who felt his soul had rotted away, finds a new purpose in fighting for her, while Caster finds the sincere and devoted partner she was denied in life. She is utterly devoted to him, and this love is her greatest strength and ultimate vulnerability. She also maintains a master-servant relationship with the summoned Assassin, though it is more transactional, using him as a guardian for her territory. Her interactions with other Servants are typically hostile; she sees Saber as a prize to be captured and views most other Masters as obstacles to be eliminated.

Caster's character development is minimal in terms of change across different story routes, but profound within her own backstory. Her development is the shift from the cursed, betrayed princess of myth to the contented, if still ruthless, woman who has found love with Kuzuki. In the Unlimited Blade Works route of Fate/stay night, this development is tragically highlighted. Although she captures Saber and comes close to achieving her goals, her attachment to Kuzuki is her undoing. When Archer unleashes a rain of Noble Phantasms, Caster sacrifices herself to shield Kuzuki, dying in his arms as she professes her love. In this moment, she abandons her schemes and her wish for the Grail, choosing instead to die for the person who made her life worth living.

Regarding her notable abilities, Caster is one of the most powerful magi in the Fifth Holy Grail War, wielding sorcery from the Age of Gods that far surpasses modern magecraft. Her primary skills include Territory Creation of the highest rank, allowing her to create a magically fortified temple, and Item Construction, which she uses to create magical artifacts and dragon tooth warriors. Her most devastating personal skill is High-Speed Divine Words, a language from the age of gods that allows her to cast high-level spells with a single word, bypassing modern incantation rituals. Her Noble Phantasm, Rule Breaker, is not a weapon for combat but a conceptual anti-magic dagger that can sever any magical contract, nullify enchantments, and even break the pact between a Master and Servant. She also possesses the Golden Fleece, an EX-rank support Noble Phantasm that can summon a dragon, though she lacks the necessary skill to call it forth. While physically weak, her mastery of magic makes her an extraordinarily dangerous opponent in the Holy Grail War.