Antagonist
Character Role Analysis
Slavery
Wait a minute… Slavery is the antagonist? Why not Madam Lockton—you know, the one who beats Isabel and drugs her so she can sell her sister? If you've got these questions on the brain (or brainpan, as Isabel would say), don't worry; we'll explain everything.
To begin with, we have to consider what an antagonist actually is. If you think it's the "bad guy" in a story, you're partially right, but there's more to it than that. An antagonist is any force or character that gets in the way of the protagonist's central goal. True, Madam Lockton does plenty to complicate Isabel's efforts to find freedom and be reunited with Ruth, but there's a greater force behind her actions, one that affects the attitudes of every adult Isabel comes in contact with. That thing, dear Shmoopers, is slavery.
Think about it. When Isabel approaches Robert Finch about his sister freeing her and Ruth in her will, Robert's first impulse is to accuse her of lying because "Slaves don't read" (2.16). The alleged "propriety and civilization" (21.112) of slavery prevents Colonel Regan from helping Isabel find Ruth and allows her to be branded. The political distinctions of slavery even prevent the British official she approaches on the docks from letting her work for the Loyalist army.
Everywhere she turns, Isabel finds a road block to every possible way out of her situation—at least until she decides to break the rules and go her own way (Feel free to break into song as you imagine Isabel triumphantly rowing across the river).