Caster

She is an honored guest of the Ryūdō 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 trainee priests and even Issei. Because of her Magus ability, she has a dual role as a Servant to Kuzuki, and a Master of Assassin. Her legendary counterpart is Medea, a supposed witch from Greek mythology.

Personality
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.

Magical Skills
Unlike other Servants, she does not wield a weapon for physical combat, but her rank allows her to manipulate Mana and perform magic like a wizard, something which other Servants cannot do. Her magical ability far surpasses modern Magi, due to her technique, High-Speed Divine Words. This allows her to cast spells which would normally take minutes to cast by uttering a single word. She easily blocks Rin's Gandr shots and can teleport herself and her Master out of harm's way.

Her Noble Phantasm is Rule Breaker: Destroyer of all Talismans, an otherwise weak dagger which dispels all forms of magic that it comes in contact with. Its greatest use is in severing the bond between Master and Servant. She also possesses a Noble Phantasm called Argon Coin: The Golden Fleece, a coat of golden fleece which is capable of summoning the original Colchis dragon that guarded it after it is thrown upon the ground.

Caster does not have the knowledge or skill to summon a phantasm-beast, so it goes unused during the fifth Holy Grail War. If she did possess the proper skills necessary to summon the dragon, it would bring her up from being the weakest Servant to the middle-ranking among the Servants of the Fifth War. The dragon itself isn't strong enough to be able to make a clear difference against higher level opponents.

Plot
In the fifth Holy Grail War, shortly after her summoning, she kills her original Master within a few days, and is later taken in by Sōichirō Kuzuki at the Ryūdō Temple. To that end, she takes the souls of other beings to sustain herself and strengthen her defensive power, as Kuzuki is incapable of providing her with Mana.

She kidnaps Sakura by possessing her body and stabbing Saber with Rule Breaker, but, because of Saber's A-ranked magic resistance, she was only able to take away Saber's ability to use her Noble Phantasm. She then tries to sacrifice Sakura, who has latent Magus abilities, to summon the Holy Grail directly. She is then defeated by Gilgamesh's Gate of Babylon in her underground city.