by Magicovore
After some investigation (the question have already been discussed, even if no answer were given before), here's the answer for this question:"Costume" is a third type of Energy card. They function essentially like Keep card (with the twist that they can de stoled) but doesn't have the "Keep" type.
Which implies that Mimic can't copy Zombie, but you still can buy Zombie with the Opportunist effect.
I hope it's clear now, if you need extra precision, let me know.
Timothée