In a story full of social ignorance, hierarchies, and just plain cruelty, it's refreshing to see a member of the elite ruling class who sees slavery for what it is. Lady Seymour is Master Lockton's aunt, though their relationship is somewhat strained and superficial—Lady Seymour is loaded and her nephew is really just waiting for her to die so he can get his paws on her money. It also probably doesn't help that she holds some pretty progressive views about how to treat those in need, and promptly befriends Isabel.
Lady Seymour, you see, is actually kind to slaves. Check out the way she treats Isabel when she visits the first time: She notes that Isabel "could use some building up" (7.17) and gives her cookies and milk. And while this isn't exactly a grand gesture, she still does more for Isabel in the first few minutes she's there than Madam has done in the whole time she's worked for her.
Not only that, but Lady Seymour takes Isabel into her home while she recovers from the branding episode and tells her, "I find the buying and selling of children most repugnant" (24.26). Thank you, Lady Seymour, for being one of the only people in this book who knows which end is up.
She also takes Isabel's side on the issue of whether Isabel should be taking food to the prison for Curzon. While we never learn how she found out about Isabel's activities, she still declares that "It is honorable to help a friend in need" (36.25), and then warns her that Madam Lockton isn't one who agrees with that statement. Nonetheless, this demonstrates that Lady Seymour isn't afraid to fly in the face of social convention. Because of the family's Loyalist leanings, it would have been totally expected for her to turn Isabel in; instead, she simply warns her to be careful.
Lady Seymour's ultimate demonstration of her feelings for Isabel, though, occurs at the end of the book, when she essentially serves as an accomplice to Isabel's freedom. Even though she's ill and near death, she allows Isabel to take her money and uses all her strength to tell her to "Run" (44.26). Earlier, Lady Seymour expresses guilt to Isabel about not following through on her desire to buy her from the Locktons. By aiding her get away, she works to make up for this fault.