Talking about Donna of course always reminds me of Jamie.
Jamie was the Second Doctor’s companion, from his second-ever adventure to the end of his incarnation. For three seasons and 116 episodes – more than any other companion – Jamie was the Doctor’s best friend, his foil, his staunch and loyal defender. He stayed on while Ben and Polly and then Victoria departed. He was never the smartest person in the room, but frequently the most fearless, and he never let the masses of things he didn’t understand slow him down.
Jamie finally left the Doctor’s side when the Time Lords took his memories and plopped him back in the Battle of Culloden as if he’d never been away. They didn’t violate his mind to save his life; they did it to punish him for befriending the Doctor, and to punish the Doctor for his “interference.” While Donna and Zoe were returned to safe and comfortable – if ordinary – circumstances, he was left in a war zone, in a time when young men’s lives were cheap. After all he’d been through at the Doctor’s side, he was left with nothing: no family, no defense mechanism, and no memory of the greatest adventure of his life.
The Doctor never forgot Jamie. The Fifth Doctor briefly mistook Adric for him. The Tenth Doctor used his name while traveling in Scotland. But like Donna, Jamie can never remember.