In the book Chains, who are the main characters and why are they important?

The main protagonist of Chains is Isabel. The entire story revolves around her and the events that impact her. The story begins with her thinking that she and her sister are going to be freed from slavery, but that quickly unravels. The rest of the book is about her struggle to keep herself and her sister safe and together while finding a way to regain their once-given freedom. Her story is woven together with Revolutionary war events that are happening in New York.
In terms of importance to Isabel, the next major character should be Ruth, Isabel's sister. Ruth suffers from epilepsy, and she probably has some other developmental issues going on too. She's important because she needs protecting. Isabel is Ruth's sister and protector, and she takes her job very seriously. Isabel will even take the beatings meant for Ruth.

Better me than Ruth, better me than Ruth.

Master and Madame Lockton are important because they own Isabel and Ruth. They are also important to the story because they are Loyalists who actively work against the Patriots. Isabel uses her proximity to the Locktons to spy on them and help the Patriot cause.
Curzon is the character who convinces Isabel to work as a spy. Their relationship as friends becomes important enough to Isabel for her to risk her freedom in order to save him at the end of the book.
I would include Lady Seymour as the last main character. She's important because she is a wealthy Loyalist who is also kind to Isabel.  n fact, Lady Seymour plays an important part in helping Isabel escape and gain her freedom by allowing Isabel to take her money.