4. Harry Sullivan
UNIT's medical officer sort of started as a prototype Mickey the Idiot, eager and enthusiastic without really understanding what was happening. He was skeptical about the Doctor's claims that the TARDIS could go anywhere in time and space and thought the Brigadier and Sarah Jane were merely humoring UNIT's extremely eccentric scientific advisor. In turn, the Fourth Doctor loved to tease him about his clumsiness. But even though his skepticism and ineptitude exasperated the Doctor who wouldn't allow him anywhere near the TARDIS controls after he twisted the helmic regulator too hard Harry was a valued member of the TARDIS crew. His courage and determination often helped him find a way out of potentially fatal situations, and these qualities helped him earn the Doctor's respect and friendship. He chose to remain on Earth in The Android Invasion and resume his duties as a UNIT medical officer, but we never have a chance to see how his travels shaped his life and work with UNIT. Apart from Jo Grant, Harry was one of the rare UNIT members to travel full-time in the TARDIS (she had been grounded while Liz Shaw was the companion, and Brigadier Lethbridge-Stewart, Sergeant Benton, and Captain Yates were primarily Earth-bound). Prolonged exposure to different times and cultures in addition to the Doctor's disdain for unwarranted violence must have changed how he viewed UNIT--maybe he was one of the people who helped change it into a more peaceful organization.