The wonderful Alex Kingston brought us my second favourite companion of all time River Song!  Rivers origins are a complete mystery to us for quite some time after she first appears in 2008. Because River Song is a time traveller herself, her adventures with the Doctor occur out of synchronisation; their first meeting (from the audience's perspective) is his first and apparently her last. Kingston plays her in 15 episodes, as River becomes a companion, romantic interest and eventual wife of the Doctor in his eleventh incarnation portrayed by Matt Smith. She has interaction with the Tenth, Eleventh and Twelfth Doctors and I wish she would come back!