Ace in the Hole says, "Set aside that die after it is resolved or removed."
Does that mean it can sit out through the upkeep phase? The upkeep phase section of the RRG says for each player to perform a number of steps. Step 2 is "Returns all of the dice still in their dice pool to their matching cards."
The RRG doesn't use the word "remove." Also, there's no card in play to return it to. So if I never resolve it, and my opponent doesn't manage to remove it before the action phase is ended, what happens to the die?