joker in the pack Definition

1. cards.
An extra card in a pack, usually bearing a picture of a jester, used in certain games.
2. A cheerful person, always full of jokes.

Thesaurus: buffoon, jester, character, clown, comic, fool, prankster, wag, wit, kidder.
3. colloq
An irresponsible or incompetent person.
4. colloq
A person.

Someone or something whose effect on a situation is unpredictable.