Mrs. Jewkes is the housekeeper in Lincolnshire, which is one of Mr. B's estates. When we first meet her, she has a pretty shaky moral compass; she believes that her duty to Mr. B trumps any moral obligation to protect Pamela from his attacks. (You also get the feeling that she'd be happy to let him have his way with her, but that's another story.)
As she puts it to Pamela, "he is my Master, and if he bids me do a Thing that I can do, I think I ought to do it, and let him, who has Power, to command me, look to the Lawfulness of it" (36.67). In other words, she sees it as her job to obey—and his job to worry about the morality.
Pamela paints a truly grotesque picture of Mrs. Jewkes's physical appearance, although we get the feeling that she's a little prejudiced by the whole aiding-and-abetting thing:
Now I will give you a Picture of this Wretch! She is a broad, squat, pursy, fat Thing, quite ugly, if anything God made can be ugly; about forty Years old. She has a huge Hand, and an Arm as thick as my Waist, I believe. Her Nose is flat and crooked, and her Brows grow over her Eyes; a dead, spiteful, grey, goggling Eye, to be sure, she has. And her Face is flat and broad; and as to Colour, looks like as if it had been pickled a Month in Salt-petre: I dare say she drinks!—She has a hoarse man-like Voice, and is as thick as she's long; and yet looks so deadly strong, that I am afraid she would dash me at her Foot in an Instant, if I was to vex her.—So that with a Heart more ugly than her Face, she frightens me sadly; and I am undone, to be sure, if God does not protect me; for she is very, very wicked—indeed she is. (38.6)
We think it's worth putting in this whole description, because this is how Richardson does character. In eighteenth-century literature, convention called for the outsides to match the insides: if someone was ugly on the outside, like Mrs. Jewkes, then you can be pretty sure she was ugly on the inside, too. (And if someone is beautiful on the outside, like Pamela, she's probably going to be beautiful on the inside, too.) We know Mrs. Jewkes is a nasty character because she helps Mr. B, slaps Pamela around, and is generally bad-tempered and corrupt, but we don't even need to see her do anything to get the picture. All we have to do is look at her ugly mug.
It's not all bad, though: once Mr. B's behavior starts to turn, so does Mrs. Jewkes's. Don't get too excited, though: we (and Pamela) are pretty sure that Mrs. Jewkes only starts buttering Pamela up because the girl is now her boss. As Pamela says in her own words:
And I could see by her Manner, that she was a little struck inwardly at some of her former Conduct to me. But, poor wretch, it is, I must fear, because I am what I am; for she has otherwise very little Remorse, I doubt.—Her Talk and Actions are intirely different from what they us'd to be, quite circumspect and decent; and I should have thought her virtuous, and even pious, had I never known her in another Light. (91.4)
Though Pamela is probably never going to pal around with Mrs. Jewkes or invite her to her to a prayer group, they do end up being on pleasant enough terms. In fact, Pamela even asks her to be one of the few people present at her marriage to Mr. B. We'd say that's a pretty decent vote of confidence.
We do have a follow-up question for you to chew on, though: Pamela apparently believes that Mr. B can change deep down, even though she once knew him "in another Light." So, why does she not believe that Mrs. Jewkes can change?