Born: c. 1509, Died: c. 1537
Known for: Being Henry's third wife; bearing his first official male heir; not being executed. (Source)
Henry VIII is obsessed with the virgin/whore dichotomy. He is attracted to virgins, but he wants them to seduce him like whores; after that, he gets tired of them, dismissing them as skanks. In other words, he's like many men today.
Henry, with his control issues, gets sick of Anne, so he does a total 180 and turns his affections to Jane Seymour, a walking doormat of a woman who would fall over in a light breeze. "We're all whores compared with Jane" (47.359), Mary quips.
Jane Seymour practically has a halo over her head.
She reminds Henry of Katherine, which is one reason why he is so interested in her. He wants to turn back time and atone for his mistakes. By "mistakes," we mean everything having to do with Anne. Katherine had a backbone, though, which Jane lacks. That makes her even more appealing to Henry, whose number one pet peeve is a woman who talks back to him…or talks at all.
Jane is an interesting foil to Mary, which means that her presence allows Mary to look back at her own time in court and see how far she has come:
I thought that to be a Seymour girl must be very like being a Boleyn girl, when your father and your brother thrust you toward the king and you have neither the ability nor wisdom to race away. (47.632)
Mary now has the ability and the wisdom to get away. In contrast with Jane, Mary's growth as a character is especially impressive.
It's important to note that Mary is empathetic toward Jane, but not sympathetic. She still hates Jane and will defend her sister against Jane whenever she can. If Jane were more aggressive, she and Mary might even get into some fierce fights—but meek Jane takes everything Mary has to say to her with a timid nod of the head.