Carol Richmond was a crewmember on Captain Maitland’s ship. She and John were set to be married.
She, John and Captain Maitland were held prisoner on their ship by the Sensorites. Once the First Doctor and his companions arrived and negotiated with the Sensorites, she and John were brought to the Sense Sphere so the Sensorite scientists could cure John. She was allowed to return to Earth. (The Sensorites)