Vicki is pretty much a replacement for Susan who exited Dr Who in the previous story, The Dalek's Invasion of Earth. In the next story, The Rescue, The Doctor, Ian, and Barbara discover Vicki as a castaway on the planet DIDO. She's a human from 24th century earth and after a few adventures the TARDIS crew adopts her. Vicki was a crew member for pretty much all of 1965 ending with "The Myth Makers."
Vicki is played by Maureen O'brien. She was born on June 29, 1943. Before Dr Who, Maureen had acted in a wide variety of roles, but after it she started writing mystery novels and only came back to acting later.