Well considering the dark knight from FFX-2, they had debuff-spells etc, meaning that Virus would fit to this category quite well. Eye for an Eye would be a nice utility for a dps.
Of course their soul eater will be their main source of damage, I'd say that Marauder (Even if he has an Axe and not a great-sword) would fit much better as base class than Gladiator due to the Power/Blood theme.
The second cross class tree could easily be lancer, the pugilist one wouldn't fit too much because of Mantra.