The Doctor may sometimes act like the most important person in the universe, but they know that's not really true. They need people to stop them, they need people to remind them to be better, and they need a charismatic Roman marble trader to lend them their face.
Basically, the Doctor can't save the universe alone. That's why they always travel with companions, or – in a pinch – enlist some supporting characters for a one-off special. The near 60-year history of Doctor Who is full of characters who have had monumental importance to the wider fictional universe through tiny acts of kindness or huge gestures.
Sometimes, minor Doctor Who characters also leave lasting impressions on the show's creatives. There are many characters whose backstories have been greatly expanded upon in spinoff fiction or future episodes. This collection of characters will mostly relate to information revealed onscreen, rather than in Big Finish audios, novels, or other expanded media.
The following is merely a handful of the seemingly peripheral or disposable characters that have left a lasting impression on both the fictional and real worlds of Doctor Who.
10. Bill's Housemate Harry
As featured in Series 10's Knock Knock, Bill's housemate Harry seemed like a nice enough young man who was destined to be bumped off by David Suchet's creepy landlord. It's a good job that Harry survives his ordeal in the end, because an old friend of the Doctor's would never have forgiven him.
That's because writer Mike Bartlett revealed to Doctor Who Magazine that it was his intention to make Harry the grandson of Fourth Doctor companion Harry Sullivan! The line was ultimately cut from the finished script, making it hard to figure out if this was the same grandfather who tried to steal a bit of the Great Wall of China with his boyfriend.
Regardless, it adds a completely new dimension to the story, as the Doctor and young Harry teaming up now has more emotional resonance in retrospect.
He gets quite a horrific "death", being devoured by the Dryads after putting his foot through a rickety step. Imagine the Twelfth Doctor having to pay a visit to his old pal Harry Sullivan to inform him about his grandson.
Thankfully, the Doctor and Bill convince Eliza to end the charade that she and her son have been maintaining, reconstituting all of Bill's friends and saving the Doctor from a very awkward conversation.