There is never any outright admission made that Rhett and Belle had a sexual relationship. There are references to her son though that might suggest that Rhett is the father of that boy. In chapter 13, Belle gives Melanie money wrapped in a handkerchief with Rhett's initials on it. Later, when Frank, Ashley, and some others try to rid the area of a camp of vagrants and the local military presence finds out about it, there is a skirmish between the opposing sides. Frank is killed and Ashley is wounded. Rhett, having heard about the plan that Frank and the others have concocted, tries to intervene but he is too late. He's not too late, however, to help Ashley hide out and provide him with an alibi. Rhett takes Ashley to Belle's and she readily helps out. Later still, when Rhett and Scarlet are married and they are having problems, it is Belle that Rhett turns to for comfort and friendship. The relationship is only hinted at, but it seems that there is a pretty deep relationship between Rhett and Belle.