Melody Pond

River Song is a human who lives in the 51st century. Sometime before she met the Doctor in his personal time-line, she traveled with him. When he met her on the abandoned planet called The Library, it was the first time he had met her. When River Song first arrived on the Library, she was with an archaeological team that she was assisting. This team was led by Strackman Lux on his mission to find the Library, and what had happened there 100 years previous. She met the Doctor and Donna when Lux and his crew hacked into the Library. While the Doctor set about finding out why the Library was abandoned, River Song spoke with Donna about how she knew the Doctor.

But it seemed the Doctor of River Song's future had told her Donna's fate. One of her colleagues, Miss Evangelista, was found dead shortly after walking into one of the shadows made up of the Vashta Nerada which subsequently killed her. Her Data Chip had not been switched off, and as Donna had made friends with Miss Evangelista previously, the last words on the Data Chip asked for Donna to comfort her.

River Song is in possession of a book which is blue, and very similar to the doors on the TARDIS. The Doctor questioned her as to the contents and the origin of this book. However, River Song stated that it was "Against the rules" (the Doctor's rules), and that it contained "Spoilers"; this might indicate that the blue book is in fact the Doctor's biography, written some time after his death later on in his own personal time-line.

When Proper Dave is possessed by the Vashta Nerada, it is revealed that River Song has a Sonic screwdriver which she claims the Doctor gave to her. As the possessed skeleton of Proper Dave chased River Song and her colleagues, the lights in the Library go out from the other end, trapping the group and the Doctor in between two different swarms of the Vhasta Nerada. (DW: Silence in the Library/Forest of the Dead)

The name River Song may be an intentional play on words. If the Ood's reference to the Doctor's song refers to his feelings of loss over Rose's departure, then it's possible that the song of love between the Doctor and this wandering archaeologist is being "reverse sung".