There have been several versions of K-9 (Mark I, Mark II, Mark III, Mark IV, as well as another version on K9, an Australian spinoff series). When Leela decided to stay behind on Gallifrey rather than continue traveling with the Doctor, Mark I stayed with her. Mark II stayed with Romana when she left the TARDIS to stay in E-Space rather than return to N-Space and become President of the Time Lords. The Doctor sent Mark III to Sarah Jane, care of her aunt, and it’s this version we meet in “School Reunion”. After K-9 Mark III sacrifices himself to allow the Doctor, Sarah Jane, Rose, and Mickey to escape, the Doctor leaves K-9 Mark IV with Sarah Jane when he leaves in the TARDIS.
Sergeant Benton is an officer working with UNIT. Though he does share two stories with the Second Doctor, one as an unnamed soldier, he functions primarily as an ally for the UNIT stories of the Third and Fourth Doctors. We know little about his past, but he does serve as a contrast to Capt. Yates, implying he had a decidedly less privileged upbringing.
Romana is a Time Lady first assigned to travel with the Doctor by the White Guardian* (*Spoilers!). After traveling with him for the Key to Time season, she chooses to regenerate, with fans distinguishing between these incarnations by calling her Romana I or Romana II. Just like the Doctor, she has a distinctly different personality in her new regeneration and she and the Doctor have a distinctly different rapport.
Dr. Harry Sullivan is a surgeon working for UNIT when he’s called in to check on the Third Doctor after his fatal dose of radiation at the end of Planet of the Spiders. He arrives just after the Doctor regenerates and is tasked by the Brig to give the Fourth Doctor a physical, leading to one of the most entertaining opening scenes with a new Doctor in the series’ run. Afterward the events of Robot, Harry is swept away in the TARDIS for the Doctor’s next adventure, one TARDIS trip that lasts the next five stories.
Tegan is an Australian on her way to Heathrow to start her job as a stewardess for Air Australia when her aunt’s car gets a flat tire and she enters the TARDIS thinking it’s a police box, intending to call for help. While she’s wandering through the TARDIS, the Master kills her aunt, causing her to be swept into the Doctor’s struggle against him.
Leela is fierce, bold, and combative. She’s surprisingly flexible to life aboard the TARDIS, given her background, and she bonds quickly with the Doctor, who takes it upon himself to sand down her rough edges. As a warrior, her initial instinct in any given situation is to quickly and permanently shut down threats. This makes her a natural counterpoint to the Doctor and while she grows tremendously due to her travels with him, she never entirely abandons her more physical approach to conflict resolution. Leela is a woman of few words and an ally to the oppressed. She has little patience for weakness however, finding those unwilling to help themselves, relying instead on others for protection, obnoxious and unworthy of assistance. She’s incredibly determined and self-motivated, jumping aboard the TARDIS after the Doctor denies her request to travel with him, and despite these many elements to her gruff exterior, Leela can be sensitive and emotionally vulnerable from time to time.
The Fourth Doctor can perhaps best be described as alien. After the very human Third Doctor and UNIT era, this new wide-eyed and unpredictable Doctor is a dramatic change and while each Doctor to this point is brilliant and clearly the smartest man in whatever room he enters, this incarnation is the first to embody that brand of genius that can keep any number of seemingly random threads whirling in their brain at a given moment, jumping between them at will and only later revealing to the rest of the room how they’re connected. He has a manic energy and bluster that seems endless as well as a penchant for bickering with or teasing his Companions, particularly Sarah Jane and Romana I and though he can be deadly serious, this Doctor is most likely to be found with a wide grin on his face and mad schemes percolating behind his eyes.
Sarah Jane is a journalist sneaking in to UNIT with her virologist aunt’s credentials when she meets the Doctor. She’s eventually caught, but she leaves an impression and ends up replacing the recently-departed Jo Grant as the Companion. After traveling with the Doctor, he drops her off in the wrong city, if not the wrong time, leaving her behind when faced with a summons to Gallifrey. Sarah runs into the Doctor again much later, allowing her to meet Rose and Mickey and get some much-needed closure.
Adric is a child genius, with a focus in math, from the 32nd century. Though he looks outwardly human, he’s actually Alzarian, or from Alzarius, a planet in E-Space (a parallel dimension). After working with the Doctor and Romana to help the Alzarians take to the stars, Adric stows away aboard the TARDIS and joins them on their adventures.
Romanadvoratrelundar (the Doctor dubs her Romana) is a Time Lady who is foisted upon the then-Companionless Fourth Doctor (and vice versa) by the White Guardian* to help him in his search for the Key to Time, a highly powerful item broken into six parts and scattered throughout the universe. She’s fresh from the Academy and incredibly intelligent, more so even than the Doctor. Romana is also much younger than the Doctor and, having just left Gallifrey for presumably the first time, he’s a far more experienced adventurer.