5. Spike - Buffy The Vampire Slayer
When Spike first popped up on Buffy the Vampire Slayer, he was a punk vampire who came to Sunnydale with his delightfully mad girlfriend Druscilla. Just like the audience, he was so not impressed with the Anointed One, and was not prepared to take orders from anyone. He was only supposed to be in a few episodes, five at most. But he made such an impression on Joss and company that they couldn't face the idea of killing him off after such a short run on the show. As they quickly realized, there was so much more they could do with the character. He didn't die in the second season. Or the third, or the fourth, or any other seasons on Buffy (well, except for the series finale, and even then he didn't stay dead for long, did he?). He even went on to star in the fifth and final season on Angel. You basically can't get rid of Spike, he's like cockroaches or Christian fundamentalists. Although Buffy/Angel was the first real ship that we got from the show, these days Buffy/Spike has been growing strong. All in all, not too bad, considering that the cheeky little vampire was only really meant to appear in a handful of episodes.