3. Jamie McCrimmon - Doctor Who
When Frazier Hines was hired to appear on Doctor Who back in the late 60s, he was only really supposed to be in one serial: The Highlanders. But he had such great chemistry with Patrick Troughton, the Second Doctor, that they decided to extend his run on the show indefinitely. Instead of swanning off at the end of The Highlanders as originally planned, he hitched a ride in the TARDIS. And although no one knew it then, he was about to become one of the most enduring companions in the popular show's fifty year run. A young Scottish man from the 1700s, Jamie was loyal, inquisitive, and brave. He went from what really should have just been a guest appearance and became one of the Doctor's best friends. You can tell in all of their scenes together, even when there's the more traditional female companion present, that the Doctor and Jamie are the ones who really work well together as a team. He travels through time and space with Patrick Troughton's Doctor for twenty serials, and even returned for two anniversary specials. It's an iconic performance, made all the more impressive considering that he was only meant to be a one-off character.