Melody Pond

River Song is a human Archaeologist.

She is shown, and assumed to have an intimate relationship with the Doctor, being the only person to know his real name.

Biography
Professor River Song is an archaeologist from the 51st century who was highly familiar with the Doctor. The first time the Doctor met her was actually the day she died on the abandoned planet called The Library. At this point, she had already known him for quite some time and had travelled with him in the TARDIS as his Companion and more.

Before Doctor Song arrived at the Library, she sent a message to the Doctor via his Psychic paper requesting help. This team was financed by Felman Lux on his mission to find the Library, and what disaster had happened there 100 years previous that had caused 4022 people to disappear. She met the Doctor and Donna when Lux and his crew hacked into the Library, and was surprised to find that it wasn't "her Doctor". The message was meant for a later version of the Doctor, but ended up being the reason for them meeting in the first place.

She and the Doctor appeared to be very familiar with an implied level of intimacy shared between the two, apparently marriage. She had in her possession a worn and battered book which is blue and has a cover that looked very similar in appearance to the doors on the TARDIS. When the Doctor questioned her as to the contents and the origin of this book, she rebuked him, stated it was "Against the rules" (the Doctor's rules) and that it contained "Spoilers" to the Doctor's own personal future. She does reveal that she's seen him turn back entire armies on his own, and then walk back into his TARDIS while snapping his fingers to open the doors, something the Doctor claims a TARDIS can't do. It appeared the book was a journal of the Doctor and River's experiences together in his own personal future.

While the Doctor set about finding out why the Library was abandoned, River Song spoke with Donna Noble about how she knew the Doctor. After River realized his Companion this time was Donna, she was shocked to even see her, which causes Donna to question her own fate in the future since Prof. Song did not know who she was.

When Proper Dave is stalked by the Vashta Nerada, it is revealed that River Song has a Sonic screwdriver which she later claims the Doctor gave to her. Later when running from the Vashta Nerada, River grabs the Doctor's hand and yells "Run!"

After they are safe, River talks to the Doctor and reveals he gave her his own future sonic screwdriver with more features than his current one. The last time she saw him, he showed up on her doorstep with a new haircut and a fresh suit, and took her to see the Singing Towers. He cried that day and gave her his screwdriver.

She realizes she has to earn his trust quickly as he keeps wondering who she is, and whispers something to him that causes him to look at her with shock, she had whispered his real name, apologizing profusely for revealing a part of his own future to him.

Later, when she realized the Doctor was going to sacrifice himself to save Donna and the 4022 people trapped on the Library, she punches him out to take his place. After he wakes up handcuffed and unable to stop her, she reveals that he must have always known how she was going to die. That he cried that last time she had seen his future self because he knew her time was coming then. She pleaded with him to not try and change history, not to change one moment of what is to come for them. Moments before she sacrifices herself, the Doctor pleads with her to tell him why she knows his name. That he would only tell her his name for one reason, that he had only one time he could do that. She tells him to shush before reciting his own rule about "spoilers".

After saving all the trapped people, the Doctor realizes his future self must have given her his sonic screwdriver for a reason. Inside he finds a data chip, which contains River's Data Ghost. The Doctor rushes to save her by downloading her into the virtual world contained in The Library's core. The data chip also managed to save the data ghosts of all of River's dead archaeological team, so she would have company in the virtual world.

As the Doctor leaves the Library, he snaps his fingers to open and close the doors of the TARDIS, smiling when they comply. It is implied that the Doctor supposedly leaves River's diary and screwdriver in the Library as he heads off to the next chapter of his journey, looking forward to meeting River again.

It is rumoured that River will appear in either The Stolen Earth and/or Journey's End as the Doctor describes it as the end of the world, which River says she has been to.

Trivia
River Song is a reference to Mark Twain's Huckleberry Finn.