River Song (The Time of Clara)

"Goodbye, sweetie."

- - River to the Eleventh Doctor, "The Name of the Doctor"

River Song was a former companion/ally of the Tenth, Eleventh, and Twelfth Doctors. River died in "Silence in the Library"/"Forest of the Dead" saving many people, and returned to say goodbye to the Doctor in "The Name of the Doctor". She will somehow return in Series 1.

Death
River Song died saving the Doctor and Donna Noble, and other people "saved" by CAL in the Library. Her final words to the Doctor were "Spoilers." ("Forest of the Dead")

Post-mortem
Afterwards, River Song appeared to Clara Oswald, Madame Vastra, Jenny Flint, and Strax in dreams. When the dream was cut due to a subsequent attack by the Whispermen, River manifested in Clara's mind. She assisted Clara in saving the Doctor at Trenzalore. She finally parted ways with the Doctor soon after. ("The Name of the Doctor")

Personality and traits
River was a very strong woman, and truly loved. Though she believed the Doctor did not love her ("The Husbands of River Song"), she soon learned from the Twelfth Doctor that he did.

Behind the scenes
River Song is portrayed by Alex Kingston.