Lady B is Pamela's previous mistress; Pamela was Lady B's waiting-maid until her death (which occurred just prior to the opening of the novel). Lady B is the mother of Mr. B and Lady Davers.
Lady B appears to have been a kind mistress—in fact, on her deathbed, she was preoccupied with making sure her staff were going to be taken care of after her death. She was kind to Pamela and helped further her education, and Pamela has nothing but snuggly feelings toward her. Plus, she had some sweet clothes.