Jackey is Lady Davers's nephew. At first, he seems like a real jerkwad, joining his mom in taunting and insulting Pamela when they believe she is Mr. B's mistress, and even pulling his sword at Pamela when she attempts to escape from their company.
One good thing about Jackey: he opens himself up to some really good zingers. Maybe because she spends a good portion of the book imprisoned and taunted by people she can't really let loose on (e.g., her boss, her new sister-in-law, etc.), Pamela almost seems stoked that she doesn't have to show him respect. At one point, she addresses him as "Tinsel'd Toy" (92.66), which is definitely going to be our new insult.
However, once he realizes that Pamela and Mr. B are actually married, he changes his tune and apologizes for his behavior—and he encourages his aunt to do the same. Everyone ends up on good terms with him at the end, and we're crossing our fingers that Mr. B's good example will keep him from molesting any more servant girls.