The statement is a metaphor that describes Isabel's personality. When people think of lions, they tend to connote things like beauty, strength, power, loyalty, aggression, etc. That's Isabel. She is the protagonist of the story, and throughout the novel Isabel fiercely fights for those around her and for what she believes in. For example, Isabel is absolutely loyal and protective of her sister, Ruth. Isabel even takes the madam's punishment in place of Ruth.
"Better me than Ruth, better me than Ruth"
Isabel is also willing to fight like a lion for what she knows is right. As a slave, that central focus is on her and Ruth's freedom. That means Isabel must take action in the Loyalist vs. Patriot conflict. She doesn't shy away from this fight, and Isabel takes an active role. She is brave like a lion.