Who is John and Annette Mandeville from Flash for Freedom! and what is their importance?

The Mandevilles own a cotton plantation in Mississippi. John hires Flashy, who is posing as a man called Tom Arnold. Flashman, on the other hand, begons a secret affair with Annette, who is described as tiny, spoiled, and imperious. When John discovers the affair, he sends Flashy to Alabama as a slave.