Zoe Harriot

Zoe was a super-intelligent person from the parapsychic unit school on Earth, which trained her to function logically and then she was sent to the Space Wheel as a librarian. She didn't like her emotionless education, so she stowed away in the Tardis after the defeat of the Cybermen on the Wheel. "The Doctor is almost as clever as I am", this was a famous quote that she said and often she was correct . Following her capture by the Time Lords, Zoe was returned to the Wheel and she remembered nothing of the Doctor or Jamie.

The Wheel (also know as Space Station W3LX88), was made to deflect meteorites from the Earth's trade routes. The Wheel was invaded by the Cybermen and their Cybermats to take over the Earth. The Doctor defeated the Cybermen with the help of Zoe Harriot.  

Zoe was in The Wheel in Space: Episode 2 to The War Games: Episode 10, plus The Five Doctors (as a "phantom"). Excluding The Invasion: Episode 3.