Melody Pond

Professor River Song was a human archaeologist. She is assumed to have an unusually intimate relationship with the Doctor, since she was the only person known to know his real name.

Biography
River Song was an archaeologist from the 51st century already highly familiar with the Doctor when they first meet (from his perspective). She was, in fact, a future companion of his who traveled with him off and on. According to her, he trusted her implicitly. River's expedition to Library was financed by Felman Lux and was chartered to find out what disaster had happened there, 100 years previously, that had caused 4022 people to "die" or disappear. When her team ran into trouble during an expedition to a deserted library, it was only natural that River should try to summon the Doctor by sending him a message via psychic paper. The version of the Doctor who turned up had not yet met her. Though bewildered, he agreed to help River and her team.

River and the Doctor appeared to be very familiar, even considering the strong bonds the Doctor forms with his companions. She keeps a worn and battered diary whose cover looks very much like the TARDIS doors, apparently a journal of her travels with the Doctor. She carried an upgraded version of the sonic screwdriver which "her" version of the Doctor had given her.

As the situation in the Library deteriorated, River realized she must prove to the Doctor that she is (or will become) someone he trusts completely and with profuse apologies, she whispers his true name into his ear. The Doctor is stunned that she would know this information, and River succeeds in winning his trust.

River died physically when she chose to sacrifice herself in order to stop the Doctor from sacrificing himself in order to save Donna and the other 4022 people trapped on the Library. She pleaded with him to not try and change history, not to change one moment of what is to come for them.

After saving all the trapped people, the Doctor realized his future self must have given her his sonic screwdriver for a reason. Inside he found a data chip, which contains River's Data Ghost. The Doctor saved her by uploading her into the virtual world contained in The Library's data core. CAL, the library's data core controlled by a human girl wired into its mainframe, also managed to save the data ghosts of all of River's dead archaeological team, so she would have company in the virtual world. (DW: Silence in the Library/Forest of the Dead)

Other information
River Song seemed to take shorter trips with the Doctor, living a more or less normal life between trips, rather than have one extended trip with the Doctor and never see him again after parting ways.

When Davros said to the Doctor that "Everywhere you go there are always deaths" River appears in a flashback, as the Doctor has flashbacks over those who have died during his travels. (DW: Journeys End)

Behind the Scenes

 * It is possible, though unlikely, that River Song is a reference to a song from the 1973 Tom Sawyer movie soundtrack.
 * At Comic-Con 2008, the character's creator Steven Moffat let slip that River Song has met other incarnations of the Doctor.
 * According to a quiz in The Sun newspaper, River Song is the Doctor's wife.