The resistance orbs are definitely situational, though they can be very useful. The crystal orb is the exception, though, since so many enemies use physical attacks, so I usually have that equipped by default (...and also because it can't be removed due to a bug). Duvalier is a frail, pathetic thing (no natural health bonus), so he needs the Blood Orb to have any degree of survivability, especially since enemies target him specifically. I chose the yellow orb over the violet one, because I found that, despite the extra 15 PsP, Duvalier's attacks are just so energy-consuming that I quickly run out anyway. The yellow orb allows him to quickly regenerate PsP without needing Luca (who can use Transfer on someone else, such as Set), which I find is more valuable than just a bigger pool. I round it out with the azure orb, because as this poll shows, everyone loves the azure orb. I don't use the green orb, though, because I already got Arman to increase Duvalier's speed. I generally find that 6 speed is good enough in most circumstances.
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