The manga is more or less a 1 to 1 copy of the VN, with this caster thing being very unclear in the Vn so the manga made up it's own little story for it, which given what we have will probably be the biggest diversion from the source material, as unfortunate as that is for some parts.

I really should read the VN, as this is way different from the movie and I'm wondering if this is more true to it. It probably is.

Poor Caster. She really deserved a better lot in life.