Dr. Flint is definitely a real person and a character in his own right. Jacobs based him on her real-life master and tormenter, Dr. James Norcom, and he seems to have been as awful as she said.
In Incidents, however, Dr. Flint rises beyond the real-life man and becomes a symbol of the cruelties and casualties of slavery. Linda talks about the "all-pervading corruption produced by slavery" and the fact that "the slaveholder's sons"—like Dr. Flint, we assume—"are, of course, vitiated, even while boys, by the unclean influences every where around them" (9).
In his sexual obsession with Linda, Dr. Flint illustrates that slavery corrupts and abuses children. His cruelty to her children shows that institution of slavery ruins family bonds. And, in his constant lies to his wife, Dr. Flint shows that slavery makes "jealousy and hatred enter the flowery home[s]" (6) of white families.
By making Dr. Flint into a symbol of everything that's wrong with slavery, especially with white slaveowners, Linda turns her own struggle into a mythical allegory against evil. Each time that Linda resists Dr. Flint’s advances, she's also resisting slavery’s violent, immoral, corrupting power.