Another companion who suffered from inconsistent characterisation, Susan Foreman remains the most important companion in the history of Doctor Who for one simple reason: If she hadn't chosen to leave when she had, they almost certainly would have stopped making the show when Jacqueline Hill and William Russell got tired of doing it. As things were, writing Susan out toward the beginning of the second series and replacing her with a different character had made the production aware that it was possible to write companions out and still have the show go on. In many ways, Christopher Eccleston deserves credit for the exact same thing, although that's another discussion. The main problem with Susan as a character is that there really wasn't one. She's written by a variety of people who all appeared to be working from different drafts of the character notes. So one week she's mysterious and alien (although not as much so as the actress would have liked), and the next she's 'kooky' and saying things like "crazy" and "I dig it". This inconsistency, combined with a general early 60s tendency for her to sprain her ankle and/or burst into tears every five minutes, led to her exit being a positive thing on many levels.