Played by Alex Kingston, River Song is the mysterious traveller of space and time who first appeared in the eighth episode of the fourth season of Doctor Who
. River Song refuses to tell The Doctor
who she is, saying she can't give him "spoilers". As of yet, who she is is still unknown, and fans everywhere have long speculated about her true identity. According to the writers of the show, this will be revealed in the sixth season.
River keeps a diary of The Doctor's life, which he isn't allowed to look at. She calls him "sweetie", which has caused viewers to wonder if their relationship goes beyond friendship. Since River and The Doctor keep meeting at different points in time and space, their timeline is rather messed up.