Jo is a young woman brought in by the Brigadier to replace Liz Shaw as the Doctor’s assistant. Her uncle, who has ties to UNIT through the United Nations, is also partially responsible for her assignment. Initially the Doctor is displeased with her appointment and skeptical of her abilities, but eventually her skill set proves to be incredibly valuable, despite being decidedly different from her predecessor’s.
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.
The Third Doctor is very scientifically minded, preferring to tinker in his lab rather than interact with his colleagues at UNIT. He’s a far more action-oriented Doctor than his predecessors, filling the Male Action Hero role himself for the first time. The Third Doctor is somewhat of a dandy, very carefully selecting his outfit and hotwiring a classic car, Bessie, that catches his eye. He can be abrupt with his Companions, but over time he develops strong bonds with them, missing them when they’re gone even coming across as a bit jealous when Jo leaves to get married. The Doctor is rather proud and his inability to operate the TARDIS is a particular sore point (the Time Lords blocked off this knowledge in his brain as a punishment for the initial conflict that caused him to leave Gallifrey). He has a contentious rapport with the Brigadier, as the Brig represents UNIT’s interests, which don’t always coincide with his own, but eventually the two develop a close friendship, with the Brigadier returning (somewhat) frequently over the course of the series to have adventures with almost all of the Doctors.
Capt. Yates is likable and charming. He has somewhat of a potential for romance with Jo, but little comes of this on the series. He’s an able officer and his eventual complicity in a scheme to return the world to the time of the dinosaurs (due to concerns over the planet’s health) comes as a complete shock. This is one of the most effective twists in the classic series, demonstrating Mike’s determination (however misguided in this instance) to save lives and fight for the common good.
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.
Dr. Elizabeth Shaw is a scientist with a physics degree and a medical degree, among others. She’s an expert in meteorites as well and is initially brought in from Cambridge by the Brig to serve as UNIT’s scientific advisor. Though initially skeptical, after encountering the Doctor and fighting the Autons, she stays on and becomes the Doctor’s assistant, after he takes the job initially offered to her.
Alistair Gordon Lethbridge-Stewart is a Colonel when we first meet him in The Web of Fear. He works with UNIT, the United Nations Intelligence Taskforce, and commands the British contingent, monitoring and defending against extraterrestrial threats. He works well with the Doctor and, when he starts his run with the Third Doctor, he’s been promoted to Brigadier, a title he keeps for the rest of his life (mostly because that’s how fans know the character- he would likely have been promoted several levels above Brigadier). Eventually, after a particularly traumatic experience, he retires, becoming a teacher at a prep school, where he once again runs into the Doctor, and afterward, he goes back to work with UNIT.