Canton Everett Delaware III was an ex-FBI agent who accompanied the Doctor during a visit to America in 1969.
Canton was an FBI agent until he left to get married, which was prohibited. Six weeks afterwards, he was contacted by President Nixon to investigate mysterious telephone calls he was receiving. There he met the Doctor, who took him with him to trace the source of the calls. (DW: The Impossible Astronaut)
In 2011, he was invited by a future version of the Doctor to the place where he would apparently die, shot by the astronaut. There he met River, Amy and Rory again, and left them gasoline with which to burn the Doctor's body. After his departure, the trio met a past version of the Doctor and searched for Canton in an attempt to change what had transpired. (DW: The Impossible Astronaut)
It is notable that when Canton showed his invitation to River Song at this time, his was still sealed and hadn't been opened. This is either a continuity error or Canton knew the time, date and location of the shooting beforehand.