I am not going to argue over the matter of your identity--frankly, after all I've seen here I would not be shocked if you are in fact Arthur Pendragon in another world. it's not my place to question things I can't entirely comprehend.

But in my reality, Arturia is the knight I have fought both with and against. Having seen her and her Noble Phantasm in combat, I can say without even a trace of doubt that she is absolutely who she says.

...honestly, this is just ridiculous enough that I might believe it.