If I had to guess, King Arthur. Judging by her Noble Phantasm, that was the Saber in my war, and she was definitely a girl.

[He shrugged, thinking over his response again. This was a delicate matter and Waver of all people knew that, so he was trying to approach with some level of tact.]

Is it okay if I ask who she was? I won't take it personally if you'd rather not answer, but there's not too much to secrecy at this point if you ask me.