River Song
"You might want to find something to hang on to..."

The first time the Doctor crosses paths with the enigmatic River Song, she is an archaeologist in the 51st century, investigating the mysterious past of the Library. In need of the Doctor's help, she summons him through a message on his psychic paper, but when he arrives with Donna, he has no idea who she is. To prove to him that one day in his future she will be the person he trusts more than anyone, she shows him the sonic screwdriver he will one day seemingly give her, and even whispers his real name into his ear. She also carries around a diary in which she has a full record of her past adventures with the Doctor - none of which he has yet experienced for himself.

Subsequently, when River ultimately sacrifices herself to save the thousands of people who have been trapped inside the Library's database, the Doctor realises that her death isn't the last time he will see her. He meets her again - in her past - when he discovers a message from her on an old artifact in a museum. At the Crash of the Byzantium, he learns that River once murdered a man, and has been granted temporary release from prison in order to face the Weeping Angels inside the Maze of the Dead. She is sent back behind bars afterwards, teasing the Doctor about their next encounter, which she remembers well - the opening of the Pandorica.

When the Pandorica does open, the Doctor and Amy discover River posing as Cleopatra in 102 AD. She left a message for the Doctor on the oldest cliff face in the universe, as a Van Gogh painting foretelling the destruction of the TARDIS was passed through time by the Doctor's allies.

On the night of Amy and Rory's wedding, River remained as tight-lipped as ever when the Doctor pressed her on her identity, warning him that everything will change when he eventually discovers the truth.