Which brand new character totally failed to mesh with an ensemble cast?

Sometimes a TV show, movie (or book series) needs to shake things up. So they add a brand new character, or replace an existing character with someone new. And... it doesn't work. What's the most glaring example of a new character who just doesn't work with the existing gang?

Please include a picture or clip of this new addition, and mention the name of the series, the character, and why they didn't play well with others. Thanks!