In Laurie Halse Anderson's novel Chains, "the lion" refers to the inner strength and determination of the story's main character and narrator, Isabel, to survive. Early in Chains, Isabel describes her family's humiliation and the horror of forced separation, noting during a slave auction that her father was not about to go down without a fight:
Poppa fought like a lion when they came for him, the strongest lion, roaring. It took five of them with hickory clubs [to subdue him].
When, later in the story, Isabel is threatening once again to run afoul of her "owner's" dictates, it is Becky Berry, Madam's housekeeper and a protector and mentor of sorts to Isabel, who comes to the young slave child's defense. Isabel describes Madam's demeanor upon coming down stairs to confront her, emphasizing the latter's complete failure or unwillingness to view Isabel as a human being. As Isabel describes Madam's perfunctory examination of her, she notes that Madam was greatly underestimating the nature of the child before her:
She did not look into my eyes, did not see the lion inside. She did not see the me of me, the Isabel.
Isabel would reference "the lion" again in the context of her emotional attachment to and determination to find her sister Ruth. Lions, of course, are of the African Savanna. Isabel is a slave of African heritage. The white slave traders and those in North America who purported to own her could not appreciate the primal need rooted deep inside of her to survive as a free human being and as the protector of her younger sister. When Madam fails to perceive the lion inside of Isabel, the latter is emphasizing the former's inability to accurately gauge the nature of her adversary.
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.