Critical opinion is a little divided when it comes to determining just who the character Beloved is in Toni Morrison's novel of the same name. I've always seen Beloved as a ghostly presence, more than a little like a revenant or zombie, but other people certainly have understood her to be an escaped slave. The two links below, one from enotes and the other from a clearly commercial site, both point to this divided opinion.
With such a novel as Beloved, I really don't expect there to be a single right answer. Rather, what each reader can hope and strive to do is to be attentive to the clues and to assemble a supported (if not absolutely certain) explanation for the title character.
Another way to look at the character of Beloved is as a metaphorical representation of Sethe's guilt over killing her 2-yr old. A psychological reading of the novel is ripe with potential comments on the fundamental nature of guilt (especially maternal guilt) regardless of racial or ethnic background. You could discuss the notion that although the narrative indicates that other people in the story - such as Paul D. and Denver - interacted with and were impacted by the character of Beloved, it could be that Morrison is actually examining the effect of one person's overwhelming and crippling guilt on the people closest to her. Again, this interpretation requires a slightly different approach to the work, but the fact that the so-called "physical" character of Beloved appears out of nowhere and disappears into the same, seems to support her as more of a symbol than anything else.