Curzon's symbol
Curzon is not just any old character in this plot. In fact, he represents a whole category of ideas ranging from free will (even as a slave, he is still clearly a force for change and improvement). When Isabel gets to know him, she gains access to a community which becomes a support group without even trying to be that. Just knowing she is part of some movement for change helps Isabel to stay hopeful. When she watches Curzon's suffering, she embodies his nature to save him, operating in the way he often does.
The sister
As a symbol, Ruth is a foil for Isabel's own suffering. She represents the abstract suffering of watching others suffer, and also the potential for bad circumstances. Ruth and Isabel are both slaves, but Ruth's slavery is more obviously tied to authority. Ruth is rarely allowed to leave her master's sight, and she is the victim of her owners' whim. Ruth's disappearance is a symbol for Isabel's own unsureness about her fate, and when her owner reveals that she intends to kill Ruth after all, Isabel is exposed to the true hatred that underpins slavery in the first place.
The brand
The letter "I" is branded on Isabel's cheek as a permanent damage of her worth. Now, she can be certain that everyone will notice her when she walks into a room. The letter "I" signifies that she is a willful or "insolent" slave who does not submit herself well to her owners' authority. The brand is an archetypal symbol that shows experience and innocence. Like Cain's mark showed his guilt, Isabel's shame refers the reader to her innocence. She is innocent in a similar way to another letter-bearer in The Scarlet Letter.
The slave in prison
When Curzon is placed in prison by the British, he becomes seriously mistreated because he is at an intersection; as a rebel, he is likely to be treated with contempt. As a slave, his humanity is constantly denied to him. The prison acts as a symbolic manifestation of what it means to be a slave. He is tortured and starved, nearly to death, and saved only by a stroke of genius by a slave who witnessed his suffering. They escape together as survivors of a horrifying world.
Escape and return
The couple don't sail away into a romantic sunset. They escape and then instantly decide to return to save Isabel's sister. This is like Harriet Tubman who escaped and then used her freedom to bring freedom to more people. This symbolic return to save her sister is also a commentary on Isabel's heroic quality. She survives a fate that makes her empathetic to the suffering of her community, and instead of using her freedom to escape, she uses her freedom to fight for more people.
