Quote Originally Posted by David75 View Post
It seems Caster has been summoned for the part of life of Gilles de Retz as a children massive serial killer.
That would be the peak of his abilities so to speak. That probably includes his deranged mental state.

As a side note, it is not clear if Gilles de Retz trully was a child murderer. But in the Grail war, heroes are summoned based on their fame, be it true or not it doesn't matter. What is important is the fame/importance of the image.
After all heroes from legends and even imaginary characters are summoned.
They come as their own selves though, if not Saber wouldn't be female.