8. Harry Sullivan
BBCThe thing that's notable about Harry Sullivan's first appearance in the series is that, uniquely among companions, he's actually been mentioned a few times before he physically appears. Even more impressively, he's mentioned in the previous season. Sadly, the reason for this unique setup is exactly the same as the reason why he ultimately didn't work out as a companion, but that doesn't make the forethought that went into his introduction any less commendable. What happened was this - Jon Pertwee was leaving the show and they hadn't yet cast the fourth Doctor. The intention was that they were going to cast an older actor, more along the lines of William Hartnell. This led them to the conclusion that they were going to need a younger male companion to do all the action bits, much like the role of Ian had been previously. The logical thing, as the Doctor was currently associated with a nice paramilitary group, was that one of their stout lads would go with the new Doctor on adventures through time and space. Unfortunately, this led them into a case of what fans of Lost might refer to as 'Nikki and Paulo syndrome'. They needed a brand new character whom everyone would accept had been around with UNIT the whole time, honest, even if he had never been seen or mentioned previously (See also: Summers, Dawn for a different spin on this.) To help paper over this crack they had the Brigadier make a phone call to the previously never mentioned Lt. Sullivan (it occurs when the Third Doctor doesn't wake up from his 'psychic spider helmet' experience for anyone who'd like to review the tape). Bingo. Instant history. Sadly for Harry they then cast Tom Baker who was perfectly capable of doing his own action bits and the whole thing was a waste of time. Still nicely handled though.