"Create symbol file" changes "character" into "Enumerated", Modelsim does not accept

Yes, you would have to change all references of it to string, or just map the internal character to parity(1) (because strings always start at index 1)

The schematic editor is pretty hobbled for functionality. The best possible fix is just not to use it, and do everything in HDL. That way you can use whatever type you want.

Using a character seems a little odd anyway. What are you doing with it?