Rhett

Rhett is not a form of Rhys. It was original the surname of the Rhett family of South Carolina, one of whose members, Robert Barnwell Rhett, was a well-known politician during the Civil War era. The family's surname is an Anglicization the Dutch name, de Raedt, and that comes from the Middle Dutch raet, "advice, counsel". [noted -ed]

Means 'advice' in Germanic when derived from a Dutch family name.

Rhett is the shortened version of Rhettman, an Old English surname. It has been in my family, from what I have been told, for over a thousand years. There are only two that still hold this name from the original family, my father and myself. Margaret Mitchell and my Uncle Rhettman Desalvo were intimate associates and the novel "Gone With The Wind" was written about the relationship she had with my uncle (he died in the early 1990's, in his 90's) and my family history in the south during that great period of civil strife among the states.

This is very unlikely to be true. Checking one of the major biographies of Margaret Mitchell, Southern Daughter by Darden Asbury Pyron, I could find no references to a "Rhettman Desalvo." Pyron's book says that Ms. Mitchell's first beau, when she was 17, was named Clifford Henry. After that romance ended, she had several suitors at once, but none of the names mentioned in the book are anything like "Rhettman Desalvo." Some of them don't have their full names given in the book, but they are called "Dan", "A.B.", and "Dr. Hudson." Full names are given for Allen Edee, Winston Withers, Jimmy Howat, Berrien "Red" Upshaw, and John Marsh. Margaret Mitchell married "Red" Upshaw in 1922 two months before she turned 22 years old; she divorced him in January 1925, and John Marsh immediately proposed to her. She married Marsh on July 4, 1925. Though Pyron doesn't think Rhett Butler is closely based on any of the men in Mitchell's life, he does mention that other critics tend to think he's based on "Red" Upshaw. In any event, it seems unlikely that there could have been any important relationship with "Rhettman Desalvo" that could have resulted in the naming of the character.

As Cleveland Kent Evans has pointed out, Rhett is a family name, borne by a famous politician during the Civil War. Maybe that's where Pargaret Mitchell got the name to use for Rhett Butler, who knows? But there's another Rhett associated with Gone with the Wind: Alicia Rhett played India Wilkes in the movie version.