Dawn Summers - Buffy The Vampire Slayer
The show was beginning to get a little boring after High School was over for Buffy, so changes were made. Joss Whedon is an amazing writer when his shows don't last very long (FIREFLY), but when a show lasts as long as Buffy did, things just tend to go nowhere.
Somewhere in the last 25% of the shows run, Dawn Summers just popped up out of nowhere. Her presence was like a big screaming force yelling "This shows gonna suck now".
Buffy was always saving her ass, and after a while I was hoping Buffy would stake her. Instead the show opted for the same crap over and over.