Josephine 'Jo' Grant played by Katy Manning
Jo Grant was the niece of a cabinet minister who had her assigned to UNIT, and the Brigadier asked her to be the Doctor's assistant. She was trained in cryptology, escapology, safebreaking and explosives.
She made a good impression on her first assignment by getting hypnotised by the Master and trying to explode a bomb in UNIT HQ.
She left to get married and go down the Amazon.