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Yep as stated they're undead, they have no vitals if we're bring in 'logic', they're just magical creatures. When you kill an undead mob you're just releasing the magic that was used to reanimate them, when you kill a dodo, you killed something living.
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They are oblivious to them because their life force has always been gone. Recently deceased still has some kind of "life" to them I guess.
I like that we can't absorb from them. You shouldn't be able to absorb from everything.
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to my knowledge drain/aspir type stuff has never worked on undead type mobs in any FF game.