1. Tegan Jovanka
The woman whom the Fifth Doctor described as being "a mouth on legs" was an accidental companion; she stumbled into the TARDIS thinking it was a real police box. It wasn't long before she realized she had found her way into something much larger than she had ever dreamed existed, but not before she accused the Doctor of kidnapping her, demanding that she be returned to Heathrow Airport so she could start her new job as an airline stewardess. But she soon learned that the TARDIS's unpredictable travels made demanding any destination pretty much useless. When the Doctor finally did return Tegan to Heathrow Airport, she found that she missed her travels. So when she unexpectedly crossed paths with him again in Amsterdam, she took advantage of the opportunity to take her place in the TARDIS once more. But the death and chaos that followed the Doctor proved to be too much for her to handle, and after a particular nasty run-in with the Daleks, Tegan ran away. She literally just ran from the Doctor after telling him she was tired of all the killing that seemed to follow them. It was one of the most abrupt departures up to that point sure, there had been companions who had unexpectedly decided to get married and leave the TARDIS, but at least they had parted on good terms with the Doctor. Here, Tegan had been in tears andignored the Doctor as he shouted after her. What happened to Tegan after that? Where did she go? Her departure left a mystery behind, something that all companion departures do to a certain extent. It's fun to speculate as to what might have happened, but sometimes--just sometimes--it would be nice to have a definitive answer. So that's what I think. What do you think? Feel free to comment!