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In all seriousness, during the fight against Adel, when Rinoa is an "enemy target" you can successfully use the devour command on her. IIRC it has the effect of "permanently" raising your max HP. Though that depends on your definition of "permanent" since doing so immediately leads to a game over (since you have to keep Rinoa alive during that fight).
Trying to use the devour command on someone in your party will always fail.
Ok, now that that's out of the way, let the immature jokes and innuendos commence.
Oh, and I'm not sure off hand, but are you sure that that message means that you can't devour the target at all? Or could it simply mean that it failed. Devour does require the target to be at low HP for you to devour it.