Jack's father's gun
Jack's father's gun that she takes with her when she leaves home is a symbol of her bravery and her standing up for herself. She is reluctant to use it against another human, but having it with her gives her strength. It is not surprising when she uses that exact gun for the first time on the person who condemned her family to death, the true beast hiding behind a mask of a man - Soneschen.
The cure
The cure is an obvious symbol of a second chance given to humanity, a hope for rebuilding a life, a hope for humanity to survive. Jonah, a cured beast, is the prime example of this hope that not everything is lost and that people like him do deserve a second chance.
Boys don't cry
This is a quote used throughout, because Jack is a girl that had to adapt herself; she had to become a boy, both in her appearance and manners. Crying is a weakness that might give out her true female identity, so she learned to toughen up. However, her brother Dean tells her at the end that boys do cry, despite the believed notion, and that it's not a sign of weakness; it is only a show of humaneness.
