Heroic Spirit of spells and magecraft. They are a class that specializes in the MGI stat, and are generally considered to be the weakest Servant.
The Caster in the story was Greek mythology’s famed witch of betrayal, Medea, and her Master was Kuzuki Souichirou.
Her role in the Saber route was just to get defeated without a fight, but in the Rin route she flourished as a villain.
She was an innocent person at her core, but after being betrayed over and over she embraced her fate, and was stained with evil.
“Fine, if that’s how it is then so be it! I’ll sink into the filth of my own volition!”
…What an earnest woman.
Anyway, despite her evil nature, she was really just a maiden in love. I can’t help but think the way she got all nervous and flustered when Shirou tried to tell Kuzuki her true colors was really cute, yeah.
Beneath her robes, she’s a strong contender for first or second most beautiful woman in the story.
She’s the kind of person that will totally devote herself to the object of her affections, but due to the trauma of having everybody she’s ever loved take advantage of her and leave her behind, she is wary about getting close to people.
Ah, it’s so disappointing. A Caster route would have been great, dammit! Especially the part where you get stabbed in the back while having an affair with Rider…
Also, this is just a digression, but Caster’s abilities as a magus are magician-level. However, regrettably, since Caster herself is incapable of acquiring “magic” she’ll be a magus to the bitter end.

Fate/side material: Fate Encyclopedia