the joker in the pack a person or factor likely to have an unpredictable effect on events.

☞ In a pack of playing cards, a joker is an extra card which does not belong to one of the four suits (clubs, diamonds, hearts, and spades) and usually bears the figure of a jester. It is used in some card games as a trump and in poker as a wild card.

1973George SimsHunters Point Fred Wheeler may be the joker in the pack. He might have got Dave involved in something wild.