Over Doctor Who's long history, many companions have joined the Doctor on his travels in the TARDIS. These companions come from many different walks of life, including humans from the past, present, and future, different aliens, and even a robot dog.
Some companions travel with the Doctor for longer periods than others, but this relationship has consistently been the most important within the show. Doctor Who is a series full of change, with the principal character regenerating multiple times over its long history, but his companions also come and go on a regular basis — and some of their departures are downright heartbreaking.
Captain Jack Harkness
Jack is one of the rare companions who has traveled with the Doctor on separate occasions. He was first a companion of the Ninth Doctor and later to the Tenth Doctor, albeit briefly. During the end of the Ninth Doctor's tenure, Jack becomes a full-time companion and travels alongside him and Rose, forming one of the best TARDIS teams on Doctor Who.
Eventually, Jack is killed by the Daleks but later resurrected by Rose before being left behind by the Doctor. Although Jack did go on to front his own spin-off with Torchwood and later reunited with the Tenth Doctor, his original exit feels unjust and is quite cruel, as the Doctor up and leaves his companion stranded.
Jamie McCrimmon and Zoe Heriot
Classic Who isn't exactly big on huge emotional exits and this is more a feature of the revival. However, the departures of the Second Doctor's companions Jamie and Zoe are quite melancholic.
Jamie and Zoe didn't leave the TARDIS by choice and neither were they killed off. Instead, most of their memories of the Doctor were erased by the Time Lords. The two genuinely enjoyed their travels with the Doctor, which can't be said for every companion, and this makes their departures quite upsetting. While there was very little buildup to their exits, it is still a sad ending for the characters.
Sarah-Jane Smith
Sarah-Jane is another companion who reunites with the Doctor years after she first left him. She is actually one of the companions who has met the most incarnations of the Doctor, having come face to face with seven versions of the Time Lord onscreen and is often considered one of the most intelligent companions of Doctor Who.
While her original exit isn't as devastating as some others, it is still upsetting due to Sarah-Jane being one of the most iconic and beloved companions. The goodbye between the Fourth Doctor and Sarah-Jane is somber and poignant and when she returns years later it becomes clear how much he meant to her. Sarah-Jane went on to get her own spin-off, and her original departure is certainly a massive turning point in the show's history.
Susan Foreman
One of the Doctor's original companions is their granddaughter, Susan, and this relationship sets her apart from most of the other TARDIS inhabitants. When Susan falls in love during her final story, the Doctor locks her out of the TARDIS as he believes she could have a better life without him.
This separation reinforces the Doctor's loneliness and his promise to come back one day is particularly poignant as he never does. When the Doctor separates from his companions, it is rare for him to see them again, which makes the line feel even more significant. Although Susan has yet to appear in New Who, it is still clear how much she meant to the Doctor as their twelfth incarnation kept a picture of her on his desk when working at a university.
Bill Potts
In the grand scheme of things, Bill is a companion for a very short amount of time, traveling with the Twelfth Doctor for just one season. However, the two form an incredibly close bond that becomes a mentor/mentee relationship which reflects those within Classic Who.
The end to Bill's time in the TARDIS is quite tragic as she is shot and then converted into a Cyberman. After being converted, Bill clings to her humanity and struggles to understand why people are scared of her, which makes her fate all the more upsetting. Eventually, she gets a second chance and is able to travel the universe with Heather; however, she goes through a lot of trauma before she gets there and still has to leave the Doctor.
Rose Tyler
Rose is the first companion of New Who and sets the precedent for many to follow. She risks her own life in order to fight her way back to the Ninth Doctor and save him and eventually falls in love with the Tenth Doctor.
Despite vowing that she will travel with him forever, Rose is forced to leave the Doctor after she becomes trapped in a parallel universe. One of the most iconic scenes from Doctor Who is the emotional goodbye between the Doctor and Rose on Bad Wolf Bay. Although she does eventually return and get a slightly happier ending, Rose's initial exit is gut-wrenching.
Amy And Rory Pond/Williams
Amy is not only the Eleventh Doctor's companion but is also technically his mother-in-law; she's married to fellow companion Rory and is mother to River Song. Although Amy begins her time in the TARDIS slightly infatuated with the Doctor, the two grow to become best friends.
Amy realizes that Rory is the man she loves and the two become a solid partnership. Eventually, she sacrifices herself to the Weeping Angels in order to be with him. Both Rory and Amy are sent back in time by the Weeping Angels, and the moment devastates both the Doctor and the audience.
Donna Noble
Donna initially rejects a place aboard the TARDIS but later becomes a companion and best friend to the Tenth Doctor. Donna is particularly important as without her, the Doctor would have died as is shown in one of Doctor Who's best Doctor-lite episodes, "Turn Left."
During the season 4 finale, Donna becomes a Time Lord/human metacrisis and while this allows her to stop the Daleks, it forces her to leave the TARDIS. In order to save her life, the Doctor has to wipe Donna's memory, meaning she has no recollection of him or her travels. Her fate is almost worse than death as she develops and becomes a better person only for it all to be reversed when her memory is wiped.