Elizabeth "Liz" Shaw (Caroline John)
Liz was the Doctor's replacement as UNIT's scientific advisor. When the Doctor returned to Earth, to begin his term of exile, she was reassigned as his assistant.
She helped repel the first Nestene invasion attempt, witnessed the reawakening of the Silurian creatures, helped to prevent the kidnap of three alien ambassadors and stopped an insane scientist from cracking open the Earth's crust.
During this latter adventure the Doctor visited a fascist parallel universe, where Liz was known as Section Leader Shaw! After only a handful of adventures, Liz eventually returned to Cambridge to continue her research.