Zoe Herriot (Wendy Padbury)
Zoe was the librarian on the space Wheel when it was attacked by the Cybermen. Thankfully this coincided with the arrival of the Doctor and Jamie, who were able to thwart them.
She hid on board the TARDIS, and pleaded with the Doctor to take her with him. Extremely intelligent, with a degree in pure mathematics - she was often able to outsmart the Doctor, although her inexperience frequently got her into trouble.
When the Time Lords finally caught the Doctor, Zoe was returned to her own time stream, as part of their attempt to fix the Doctor's meddling.