One could easily make a PLD into a DRK by simply by creating it with opposing skills, after all, DRK would be the exact opposite of PLD, the yang to it's yin which is what it's supposed to be, regardless of whether or not it makes sense to the user (I would like to take this time to thank Mr. Faulkner for that run-on sentence.)

That being said, they could base DRK atk power off of def, basically just by a change of your soul stone, essentially using the same gear. This soul stone could also change the way the vit stat works.

It all makes sense when one puts it in the right perspective.