The answer to this depends on whether you think about the plot (in which case it is Rebecca herself) or the characters we actually meet (in which case it is probably the 2nd Mrs de Winter, since we see the action through her eyes.) In the novel, and in most adaptations, we never see Rebecca. She is the late wife of a Cornish landowner, Maxim de Winter. Famed for her beauty and talents, she drowned in a boating accident but is still fondly remembered by neighbours, friends and servants. Maxim's shy, inexperienced second wife becomes obsessed with Rebecca, whom she believes her husband still loves. Gradually she learns the truth about Rebecca: the marriage was a sham, and Maxim in fact murdered his wife after a quarrel in which she claimed that she was pregnant by another man. The rest of the novel is about their joint efforts to cover up this murder and escape justice; their plans are largely thwarted by Mrs Danvers, the mad housekeeper who adored Rebecca and wants to avenge her.