Clara Oswald played by Jenna Coleman
There have been many Clara Oswalds through history from a Victorian Nanny to a Junior Entertainments Manager on a crashed star liner. When the Eleventh Doctor first met Clara Oswald she had been working as a Junior Entertainments Manager on board a star liner that had crashed into the Daleks' asylum planet. Even though this Clara had been turned into a Dalek she helped the Doctor to escape from the asylum.
The Doctor became curious when he next met a different Clara Oswald in Victorian London where she was working as a nanny and a barmaid. When he later met a third Clara Oswald in modern day London the Eleventh Doctor set out to try and discover who she was, calling her the Impossible Girl.
When the Great Intelligence travels down the Doctor's time stream, rewriting history, Clara does the same, going back through all of the Doctor's regenerations to fix the changes the Great Intelligence made.
Unlike most companions Clara rarely stays in the TARDIS between adventures. When the Doctor met her in 21st century London she was working as a live-in nanny with a family, and later progressed to being a teacher at Coal Hill School where she fell in love with a new teacher Danny Pink before his death at the hands of the Cybermen.