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River Song (played by Alex Kingston) is an archaeologist from the future who the Doctor keeps meeting in the wrong order.

He first met her on the planet of The Library, but we haven't yet seen where she first met him. So her journal is packed with information the Doctor feels he shouldn't know about yet. They're going to be very close: River knows the Doctor's real name, and she even has a sonic screwdriver of her own that she claims he gave to her. She even knows how to fly the TARDIS.

But there's a tragic element to their relationship: the Doctor knows how River dies, so when the two of them meet at the crash of the Byzantium, share that picnic at Asgard and whatever they did together at the Singing Towers, he'll have a terrible secret that he won't be able to tell her.

River has a knack for getting into trouble and knows the Doctor can help her get out of it. But as for just how much they meant to each other: spoilers!

Alex Kingston is best-known to American audiences for her role as Dr. Elizabeth Corday on the long-running drama ER, which she played from 1997 to 2004. She has starred in films such as Croupier (alongside Clive Owen) and Essex Boys (opposite Sean Bean). She has also appeared on TV series such as Lost in Austen, CSI, Without a Trace, Law & Order: Special Victims Unit, and FlashForward.