Yost McCormack

 was the flight attendant on board a spaceship the Fifth Doctor traced a Dalek signal. She was very useful to the Doctor because of her training. She incapacitated the hijacker Yan Goodrin. (AUDIO: Lightspeed)

The Doctor would eventually discover that Yost, along with many other people throughout history who looked like her, were caused by a group of Time War Daleks who — while on a mission to prevent the rise of Time Lord society on Gallifrey — created a time distortion which created alternate versions of themselves as four un-mutated Kaled people. This time distortion also caused multiple copies of these four Kaleds to appear at various points throughout time and space. (AUDIO: Effect and Cause)