Ah, Nikki and Paulo on Lost. Everyone’s favorite mistakes. (Claire’s kind of going this way, too–I’m hoping the writers will use the mystical-Aaron angle to actually give her some lines in the last two seasons.)
I might get some flack over this, but Bela from Supernatural really didn’t need more than one or maybe two episodes. It was a great premise, but the whole posh-British thing was cheesy and it seemed like she existed solely to make the boys hold the Idiot Ball for that episode. (Ruby, on the other hand, was awesome. I’m so glad she’s back. :D)
Cally from Battlestar Galactica, but maybe it’s personal bias.