In my opinion, Isabel's strongest traits are compassion, determination and bravery. All three of these traits help her to overcome obstacles.
First, Isabel is compassionate. The best example of this is that Isabel takes great risks to deliver food and supplies to Bridewell prison in order to aid the rebel prisoners who reside there after the Battle at Fort Washington. Isabel is also compassionate when she saves Lady Seymour (and her coveted picture of Mr. Seymour) from the house fire. Instead of becoming bitter in being a slave, Isabel overcomes this obstacle by serving others through her compassion.
Next, Isabel is determined (from the very beginning of the book) to keep herself and her sister, Ruth, together. When the two are separated (when Ruth is sent away to Charleston), Ruth continues her determination to find Ruth. The reader can see Isabel's determination even into the last moments of the novel when Isabel (who is finally free) and Curzon leave for Charleston in order to find Isabel's sister. Another more limited example of determination can be seen when Isabel throws the rebel note into the fire to escape detection from Mrs. Lockton. Therefore, Isabel's determination helps her overcome the obstacles of slavery, separation, and even British oppression through the examples above.
Finally, it is interesting to note that Isabel's compassion and determination nicely highlight her bravery. Isabel, of course, is a slave. Therefore, showing compassion for other slaves (as well as rebel soldiers) and disobeying direct orders from her mistress (because of her determination to find her sister and desire to aid the rebels) are very brave of Isabel. These examples of bravery all help Isabel overcome the biggest obstacle in her life: slavery.