Sarah Jane Smith
Sarah Jane Smith is tricky to categorize because she was a main companion in around three and a half seasons of Classic Who and the lead of her own spin-off. However, she was only a recurring character in the revival, which is why she’s included here. Sarah Jane was a companion to the Third and Fourth Doctors, and is often viewed as the best companion from the original show.
The Doctor dropped her back off on Earth after being summoned to Gallifrey, abandoning her to figure out how to live her life without him. She had a more open-minded approach to aliens arriving on Earth than UNIT and Torchwood, becoming a journalist to showcase the good and bad of all species.
Sarah Jane returned to Doctor Who when she ran into the Doctor while investigating Deffry Vale High School. She was only in four episodes of the revival, always interacting with the Tenth Doctor. However, a comprehensive count has her appearing in 81 episodes of Classic Who, 54 episodes of The Sarah Jane Adventures, and many other mini-series and TV movies.
Just looking at her appearances on the main show, Sarah Jane is representative of a lot of things. She was an early love interest for the Doctor (creating a foil for Rose), an abandoned companion (showing the possible future for all the Davies-era companions), and an empathetic contrast to the militaristic approach of many other former companions. Very few companions are seen after they leave the Doctor, so Sarah Jane was a much-needed example of what might happen next for the humans after the TARDIS (and thus the show) moves on.