Clara Oswald was a companion to the Eleventh and Twelfth Doctors. According to the Doctor, Clara was simply “not possible”. Though assured by the eminent empathic psychic Emma Grayling and the TARDIS scanner that Clara was just an “ordinary girl”, the Doctor couldn’t believe that to be true and thought her to be “the only mystery worth solving”. The truth was somewhere in between. She was a physically ordinary human girl from the 21st century, but one who had made an extraordinary sacrifice to save the Doctor’s life. Upholding a personal credo not to walk away from danger in the service of others, she chose to enter the Doctor’s timeline in his tomb on Trenzalore in order to reverse the damage done to his timeline by the Great Intelligence. This act allowed the time winds to break her into millions of copies of herself that weren’t quite the “real” Clara, but nevertheless capable of combatting the Great Intelligence’s attack. After her heroism revivified the Doctor, he then returned the favour, guiding the original, 21st century Clara back out of his timeline.
This act of bravery by the “ordinary girl” created the “impossible girl” who so intrigued the Doctor, largely by repeatedly dying in front of him. It meant that the Doctor met her out of synch with his own timeline. The Eleventh Doctor alone met a couple of versions of her before meeting the original: Oswin Oswald, a hyper-intelligent crew member onboard the Alaska who was converted into a Dalek; and a barmaid/nanny called Clara Oswin Oswald in Victorian London. Though her assistance mostly went unnoticed by Doctors prior to the Eleventh, she met and subtly helped them all. Significantly, she was able to attract the attention of the First Doctor and convince him to steal the “right” TARDIS when he first left Gallifrey with Susan.
Following the events on Trenzalore, Clara met the Doctor’s secret and tenth incarnations and helped convinced them to save Gallifrey rather than destroy it. Later, despite repeated attempts by the Doctor to send her home, Clara aided him in defending Trenzalore by convincing the Time Lords to grant him a new regeneration cycle, saving his life once again.