10. Spike (Buffy The Vampire Slayer)
Debuting in Season 2 and originally intended to be a three-episode-then dead villain, the vampire Spike (as portrayed by James Marsters), almost immediately became so insanely popular that series creator Joss Whedon almost had no choice but to keep him around. Spike was evil no doubt, but he was also gregarious, charming, funny, and for a creature supposedly lacking a soul, had a very varied depth of character. He cared more about his beloved Drusilla than for any grand schemes. He even helped the protagonist, the titular Buffy Summers, prevent the destruction of the world by betraying his co-conspirators because he liked
the world and thought destroying it was a bollocks idea. Over the course of the series, Spike went from scenery chewing fun villain to deadpan snarker and reluctant ally of Buffy's gang due to a chip in his head that prevented him harming humans, to going through hell to regain his soul and become a willing participant in Buffy's fight against evil. Marsters' portrayal of Spike has made him an enduring character still beloved to this day, long after the show left the air.