What do you suppose qualifies as a plot character?

This will be a post that I hope that you all will enjoy interacting with. I am hoping that it will spark or prompt conversation! Happy Chatting!

There is a never ending, plethora of characters who make appearances in Doctor Who. In the earlier episodes, the show consisted of a largely eclectic group of revolving characters who might have originally only meant to have been a temporary spot in the show but did so well as characters on their own, that they became recurring to a degree. A character to note that was like this but also almost merged the earlier Who with the newer Who and has also held up as his own character, Jack Harkness, has almost in a way a companion to the companions as stated in the article on the BBC website: https://www.bbcamerica.com/anglophenia/2011/10/a-companion-to-the-doctors-companions-captain-jack-harkness. Some of these recurring characters are only to help push the plot line along through and not actually build a character of their own but still have a major impact on the plot and the characters surrounding them, like Jenny from The Doctor’s Daughter.

Do you think that these could be seen as perhaps plot driving characters? What do you think about the possibility that Jack Harkness could be considered the “companion to the companions” as stated in the BBC article?