As a rule, if you have a black magic caster, that character should NEVER swing a weapon. If you want to put something in his/her arms, put shields there because Agility will go up just because they're there.

The game mechanics are formed this way so you cannot have a Fighter/Mage character. Unfortunately, it really rules out any possibility for a red mage. Sorry.

Keep in mind that your Strength will go down as your Spirit goes up, so it's also difficult to have a strong Fighter/Healer too.

But again, just have your magic user never use a weapon, and the Intelligence will skyrocket. (And the Vitality will plummet, consequently)