I am assuming that we are talking about internal strength here, as obviously, Kira, through her disability, is not externally strong, and is regarded as something of a liability to her tribe, as the way that her mother had to protect her from being killed at birth made clear. However, Kira definitely does not lack in internal strength, and owes her survival to the way that she is able to draw upon such internal traits and defy externally strong characters such as Vandara and Jamison. Clearly, the series of revelations that Kira discovers during the course of the novel and her ability to cope with them also show considerable internal strength. However, most importantly is her decision to not leave the community with her father and to stay with Thomas, Jo and Matt and somehow weave a new future for her people. Not to take the easy escape and to remain fighting for her right to express her art and talent in the way it was meant shows massive internal strength. Note how the following quote refers to this:
She could feel the future through her hands, in the pictures her hands were urging her to make. She could feel the broad undecorated expanse waiting, across the shoulders of the robe.
Kira's ability and inner strength gives her the fortitude she needs to reject the easy, safe option of leaving with her father and remaining to face the uncertain future under the power of the Council of Guardians, who maintain control by lies and deceit, and probably killed her mother and Annabella.