2. Harry Sullivan
Poor Harry. He wasn't needed even before he started. You see, when Ian Marter was cast as a companion for the upcoming fourth Doctor, producers had no idea WHO would be Who. It was actually assumed they would choose someone older than Jon Pertwee, so in case some muscle was needed, they would need a young, fit gentleman to fill the role that a decrepit old Doctor couldn't. Then they hired the fit as a fiddle Tom Baker, and Marter's days were immediately numbered. But he had a good time while on the TARDIS, and fans have always had a warm place in their hearts for him. Too bad the writers were somewhat ambivalent. After battling Daleks, Cybermen, Sontarans and the Zygons with the Doctor and Sarah, Harry seemed to have had enough, and just took a train back home. Simple. Except he inexplicably pops up again some time later in The Android Invasion. And then just goes away again at the end of the episode. An ignoble end to a noble friend.