Joe sounds like someone with special and specific skills, that was brought in to fill a specific purpose, without other people knowing.
For instance here is a situation of a 'ringer' being brought it.
Say everyone in a town knew a certain horse never won races (let's call him Pokey), so people betting would never bet on him. One day, that horse's owner went out of state and found another horse named Lightning that looked *exactly* like his loser horse Pokey but was faster than any horse around. So that owner brought him back to his own town, entered him in a high-stakes race, but told everybody it was Pokey. Then he bet a huge amount on him to win. That horse Lightning is a ringer - he looks like one thing but is completely another.