That's just it- she's always refered to as either "the first Mrs. DeWinter" or "Maxim's second wife". The book is told from her point of view. When the film was being made, the writers considered calling her "Daphne" after the book's author Daphne duMaurier, but David O. Selznick rejected this, explaining that the second Mrs. deWinter should be nameless to reflect her lack of a persona as opposed to the opressive legend of Rebecca, the impossibly beautiful lady who held sway over all in her orbit. In the script, she is called "I".
I'm surprised you didn't notice that Olivier had his temples 'touched up' in the movie to show age. Also, it may have been mentioned in passing, but the second Mrs. deWinter is an American. This explains why such American actresses like Loretta Young, Anne Baxter (who almost got the role- get the Criterion DVD to see her extensive screen tests), and Margaret Sullavan tested for the part. Joan Fontaine had the best of both worlds: born to British parents in Tokyo and raised in L.A. with older sister (and lifelong rival) Olivia deHavilland, so she could play both American and British characters with ease.
[spoiler]One big change had to be made in the movie to satisfy the censors. In the book, Rebecca goads Maxim into shooting her, and he does- right through her heart. Under the 1934 Production Code, people weren't allowed to get away with murder, so in the film Rebecca trips and falls, fatally hitting her head on an anchor just as Maxim is about to shoot her.[/spoiler]
