River Song was created by Steven Moffat and is portrayed by actress Alex Kingston.
River Song first appears in 2008 during Season 4 of Doctor Who in an episode titled "Silence in the Library." Herein, she meets the tenth incarnation of the Doctor, portrayed by David Tennant, and his then-companion, Donna Noble. A professor of archaeology, Song sends a message to the Doctor via his psychic paper, summoning him to a planet-wide library in the 51st century. However, when the tenth incarnation arrives, she is first disappointed that he isn't "her Doctor," and then shocked when she realizes that he is an earlier incarnation than their first meeting. During the conflict with the Vashta Narada, which carries into the following episode "The Forests of the Dead," Song whispers the Doctor's real name to him in order to gain his trust. In the end, River sacrifices her life to save the Doctor's, though her consciousness is downloaded into the library's computer mainframe, where River exists in a virtual world. The Doctor later revisits the library to communicate with River's consciousness in the seventh season.
Song next appears in the fifth season, after the Doctor has regenerated (this iteration played by Matt Smith). The Doctor and his companion, Amy Pond, help Dr. Song contend with the Weeping Angels in a two-episode long story. It is stated in the first chapter that the Doctor and Song continue to meet in the wrong order, and that Song has likenesses of all of the Doctor's faces up (presumably up until the point when they were supposed to meet). In the second half, River is revealed to be a prisoner, remanded into the custody of Father Octavian (who dies at the hands of the Angels) for killing a good man. Song confirms this, and after the Weeping Angels are dealt with, she is returned to prison.
Finally, at Battle of Demon's Run, it is revealed that River Song is actually Melody Pond, the daughter of Amy Pond and Rory Williams. Conceived in the TARDIS, Pond/Song has Time Lord DNA, apparently giving her the ability to regenerate as the Doctor does. She is stolen and trained for use as a weapon against the Doctor. After "killing" the Doctor, Song is imprisoned at Stormcage, though she frequently escapes as needed (or desired) by the Doctor.
River Song keeps a diary with a blue cover, in which her life with the Doctor has been documented, as well as other exploits that have occurred throughout her life and his future. She has also used hallucinogenic lipstick and a laser pistol with a beam that can be refined for use as a cutting torch. Honoring the Doctor's mandate that the time stream be protected, she adamantly refuses to disclose any clues to her future with the Doctor, smiling and reminding him, "Spoilers."
Alex Kingston, the actress who has brought the character to life on screen, has a long and storied career in both the theater and on the screen; for more information, visit http://www.imdb.com.