What are the special problems for writer and reader in having a ghost featured as a main character? What does beloved, as ghost, signify?
Most broadly, Beloved is the past embodied. More narrowly, she is the daughter Sethe killed to prevent her from being enslaved. But as the past embodied, Beloved enables not only Sethe but also the other characters--Denver, Paul D--to encounter stories of their sorrow and past but theirs as well. In terms of narrative device (which I think you are asking about too), Beloved's presence allows Sethe, who tends to be reticent, to tell her stories, and telling stories about the past in order to expunge from it the ghosts which haunt it--ghosts of guilt, anger,, sorrow, and pain--is what this novel is about.