The Scrappy: D-Mack. While the rest of the new cast in Season 5 at least got somewhat better with time, and were at the very least somewhat interesting, D-Mack was neither of those things. He was a wannabe gangster rapper and casanova who constantly called Lisa, who he knew had a massive crush on him, ugly and insulted her, was a general prick to everyone else, and seemed only there to see how many stereotypes they could cram into one character.
Seasonal Rot: Season 5, which did away with every character except Breanna and Arnaz and brought in four new characters to replace them, threw in a bunch of random transitions, and generally lost what humor they had. It got better towards the end of the season, but sadly the show ended before it could pick up again.