Who grades the mathematics phd comprehensive exams?
I'd think a committee is much more likely than an individual professor, but I suppose it could depend on the size of the graduate program.  Comprehensive exams are handled in many different ways in different departments (when they are held at all), so there's no way to give a universal answer.  For example, in some departments the grading is delegated to junior faculty, while in others it isn't.  If you are interested in the answer in your own department, you'll have to ask them.