Alice is a housekeeper at Manderley. She acts as Mrs. de Winter's temporary maid, and she looks down on Mrs. de Winter as lower class. So it seems that some of our narrator's fears are actually realities.
Comments from the bishop's wife get Mrs. de Winter seriously considering the idea of a costume ball at Manderley.
Captain Searle is the man who breaks the news to Mrs. de Winter and Maxim that Rebecca's boat has been found with a body inside it. He doesn't realize just how bad this news really is.
Clarice is the young woman who becomes Mrs. de Winter's personal maid. Mrs. de Winter can relate to her and sees her as a friend. She's maybe the one person in the novel who makes our narrator feel comfortable. So why don't they spend more time together? (It seems like Mrs. de W just loves her some drama.)
Frith is the butler at Manderley. He's totally polite, obedient, and reserved. We never really meet the man behind the butler.
Giles is Beatrice's husband. He's nice to Mrs. de Winter and presented sympathetically. Oh, except he might have had an affair with Rebecca.
Gran is Maxim and Beatrice's only living relative. When Mrs. de Winter visits her, she has an episode of senility or dementia and screams, "I want Rebecca […] [W]hat have you done with Rebecca?" This helps cement Mrs. de Winter's feelings of being inferior to Rebecca. Way to go, Gran.
Horridge is the coroner who rules Rebecca's death a suicide. Oops.
James Tabb is the man who converted Rebecca's sailboat and who was responsible for its maintenance and care. After investigating her boat, he testifies that someone sunk it on purpose, bringing Maxim under temporary suspicion.
While visiting Manderley, Lady Crowan pushes the issue of the costume ball until Maxim and Mrs. de Winter agree to have one. Thank you, Lady Crowan, for providing us with a major complication in the novel.
Robert is the young footman at Manderley. He's shown sympathetically, but, like the other servants, we don't learn about his life beyond his job. Just another shout-out to the importance of social class in the novel.
This drowned woman isn't really a character, but she's interesting to think about for a moment. Maybe somewhere, people are obsessing over this unknown woman, just like Maxim and the gang are obsessing over Rebecca!