Bazett Fraga McRemitz

Lancer’s original Master. Fate unknown.
She was a foreign magus dispatched by the Association, and summoned Lancer using a pair of rune stone earrings as the catalyst.
A beautiful woman dressed as a man, her abilities as a magus are top-notch.
She is an enforcer, tasked with the capture of sealing designated magi. Essentially, an existence similar to the executors of the Holy Church. She is extremely skilled in both magecraft and martial arts.
Though she probably would have ended up being a threat to Rin if she had been able to participate in the war, she was taken out prematurely by a surprise attack from Kotomine.
The reason for that is because she and Kotomine were actually old friends. Back when Kotomine was still an executor working for the Church, they often ran into each other on assignments and fought together as comrades.
So, when Kotomine, who until then had never relied on her even once, said he wanted to meet with her, Miss Bazett was secretly elated. With no reason to doubt him, she walked right into his trap.
She didn’t even have time to call out Lancer, before Kotomine cleanly sliced off her left arm from behind and stole her command spells.
Afterward, he gave Lancer two commands.
The first was to make him accept Kotomine as his Master, while the second was to make him gather intelligence.
As for what the final command spell was used for, that’s covered in the story itself.

Fate/side material: Fate Encyclopedia