Mrs. Beatrice Lacy
Beatrice is Maxim's slightly older sister. She a sympathetic character who genuinely tries to help Mrs. de Winter adapt to life at Manderley. According to Maxim, Rebecca had an affair with Beatrice's husband, and so Beatrice never liked Rebecca. Beatrice is too proper and polite (in spite of her bluntness and nosiness) to ever voice any of this to Mrs. de Winter.
Still, for much of the story, Mrs. de Winter believes that Beatrice is unfavorably comparing her to Rebecca. Goes to show you: Mrs. de Winter is so obsessed with Rebecca that she thinks even the people who hated Rebecca most are constantly comparing the two women.