Yes, Denver is a more important character in this novel than Beloved.  It is Denver whose growth from a young girl into a woman that readers follow: we see Denver struggle to develop an identity that includes her mother and her father (who she's never known), then to form a relationship with the strange Beloved (whose identity Denver figures out long before anyone else seems to), then to save her mother (and herself) when Beloved begins to consume Sethe.  Beloved is a ghost, she's the past, she's the nightmare of slavery: she is almost all these things more than she ever is a true character.  Denver, on the other hand, isn't a symbol; she's a real person in this text.  She is dynamic and complex, with a past and a future.  She is the hope for the future that there can be a life after slavery.