The Doctor should not travel alone. It was the addition of Companions in his life besides his granddaughter that formed the Doctor’s moral compass, and eventually led him to becoming the protector of time and space we know today. Without a Companion, especially in New Who, the Doctor loses his (or her) way. For the Tenth he grew cold, seeing himself as the Time Lord Victorious. For Eleven the loneliness kept him out of action.
The Companion also serves a role for the audience, someone to explain all the science fiction stuff. Even with the original TARDIS team, Ian and Barbara served the show’s original mission of science and history education being schoolteachers in those respective fields. Over time though the show became less accurate in both and just became a serialized sci-fi adventure. So the humans are the relatable ones to the alien Time Lord, they can ask the questions the audience needs to know the answers to, and gives the Doctor someone to talk to besides himself.
However, life aboard the TARDIS is not necessarily a safe one. Some Companions left on a world other than their home planet, and a few have even died. In the following video, Harbo Wholmes goes over every Companion the Doctor has had. Well, not all of them. While mentioning events in external media (books, comics, and Big Finish audio dramas among other sources of questionable canonicity) he does leave out Companions from them. So if you wondered what happened to Charlotte Pollard you’re out of luck. What disappoints me though is the one major show Companion he left out.
So who did he miss out on? He has Kameleon but NOT K-9? Really? You flaked on my favorite Companion? It’s not even because he’s a robot because, again…Kameleon is in there! K-9 has had a lot of history outside of his time in the TARDIS. Maybe we don’t count Mark III because he didn’t really travel in the TARDIS with the Doctor. He was a gift for Sarah Jane Smith to give K-9 his own show, K-9 And Company, though sadly it was a rather boring story where K-9 and Sarah went up against a plain old cult with no real powers and then just did stuff with Sarah until he broke down and had to be repaired by the Tenth Doctor, later rebuilt into Mark IV after sacrificing himself to stop the Krillitanes from using boarding school students to conquer the universe or something. I had to look up the aliens as it is.
Still, you’d think the first two K-9s, who DID travel with the Doctor deserved a mention. The original was created by Professor Marius because he couldn’t take his own dog to a space station he was working at for a brief time. K-9 (or K9 but I’m used to using the hyphen) was a computer in the form of a robotic dog, hence his name, and aided the Doctor and Leela in saving the station from an invasion by a sentient cactus. (Just go with it.) Not being able to take the robot back to Earth for reasons…the reasons being the kid audience loved the robot despite the backstage electronics issues they had…and so the end of “Invisible Enemy” was changed to make him part of the crew, a thank you gift from the Professor. However, the Doctor and K-9 tended to argue, and word is Tom Baker didn’t like the robot either, though he had no complaints with K-9’s usual voice actor, John Leeson. When Leela left the ship to stay on Galifrey with her new boyfriend, The Doctor made a second K-9. This is not where his story ends though. Children’s books would give K-9 his own adventures helping out the Time Lords, in his own ship dubbed “K-Nel”. A few of them are part of the K-9 origin DVD box set as a special feature and they were fun to read.
K-9 Mark II would travel with the Doctor as well, but only knew one other fellow Companion, Romana. When she stayed behind in E-Space K-9 went with her, to Tom Baker…I mean the Doctor’s relief I’m sure. When Romana left E-Space and returned to Galifrey in novels and Big Finish stories, K-9 II left with her and together with Leela, Andred, and K-9 Mark I would aid Romana in her various stories. The fate of this K-9 is uncertain after the Time War, though Mark I would later show up in a future London where the police were replaced by robots and the government were trying to get their hands on alien tech. K-9 would regenerate Time Lord style because the Australian company behind the K-9 series, which you can find on Saturday Night Showcase if it’s still up, only had the rights to the character, which was owned by creator Bob Baker while the BBC owns the rights to the design. I’m not sure if British rights are better or worse than ours, frankly. K-9 would join with a kid named Starkey, a scientist named Professor Griffin, and their friends trying to protect London from alien threats and the shadow organization out to steal alien stuff. I guess something happened to Torchwood.
One of these K-9s would also make a cameo in the “special” Dimensions In Time, a charity event that oddly crossed Doctor Who with the British soap opera EastEnders and would have had his own movie, Timequake, but that fell through and killed a second season of K-9. So NOW you know what happened to every major show Companion and the fact that he was left out of this retrospective makes me sad.