Canton Everett Delaware III is a character on Doctor Who, played by Mark Sheppard and his father William Morgan Sheppard. So far the character has only appeared in one story, the two part episode "The Impossible Astronaut"/"Day of the Moon", as well as a brief flashback in "The Wedding of River Song".
Delaware is an ex-FBI agent who is recruited by Richard Nixon to deal with a special problem: a series of phone calls that follow Nixon wherever he goes. Although somewhat unorthodox and rebellious, he is also a very intelligent and professional agent. His search for Nixon's caller brings him into contact with The Doctor and his other companions, and while initially wary of The Doctor and confused at the mechanics of time and space travel, he quickly adapts and becomes a key member of their team during the episode. Much later from his time line, the older Delaware, played by the senior Shepard, is invited to witness the shooting of The Doctor at Lake Silencio, and then creates a stable time loop by telling the characters to find his younger self.
Doctor Who often has a more military minded character as a foil and compliment to the usually peaceful Doctor. Canton Delaware fits this mold, not being averse to violence when confronted with a threat. However, he is not a blood-thirsty or stupid person, but is a cunning and crafty strategist, for a while working as a triple agent.
Delaware is also, as is learned at the end of "Day of the Moon", homosexual, and that was the reason for his dismissal from the FBI. The man he wants to marry is also black. Whether this is meant as a cheap piece of character depth "Macho FBI agent actually gay", or whether it is intrinsic to the character, is somewhat that remains to be seen. However, I take it as more than a cheap trick, I think that his life as a homosexual in the culture of the FBI during the 1960s is a key to Delaware's ability to be deceptive, as well to his sardonic personality.
So far, there has been no mention of Delaware returning, but he is one of the many characters that the fanbase would like to see brought back at some point.