How does character attribute advancement work in Cladun?

In ClaDun, characters can either be your Main or Sub in a Magic Circle.

Different classes have different growth depending on where they are in the circle, for example a Warrior has "Average" in both main and sub but other classes have some stats marked with "+" and others with "-".

Does this mean they gain stats faster/slower with that configuration, or that those stats go up/down upon level up and I need to swap them into the main role to prevent stats from going to zero? Does this make a big difference, or should I be more concerned with levels, equipment, and artifacts than individual stat gains?

I'm uncertain as to if stats with a - will actually go down or not. To answer your other question you should definitely switch characters between the main role and support role for reasons of stats. The stats that increase when a character is in a support role are precisely the stats needed for Main character roles. Stats raised fastest when you are a main character are primarily useful for support characters, so the most gain in the game will be had from cycling your characters in and out of the primary role when the going starts getting tougher so that you can better utilize some of your increased stats.