Evolving identity
One of the central narratives in the novel is Katniss's shifting identity. At the beginning of the story, she considers herself thoroughly a "girl from the Seam." She finds dignity in her poverty and her ability to survive it through her hunting and gathering skills. While friendly with several members of the merchant class, she identifies herself most strongly with Gale, also the child of a deceased poor miner. The stoic strength this identity has given her provides the philosophy she thinks will help her succeed in the Games.
However, through the adventure, Katniss is forced to question both her identity as a "girl from the Seam" and her stoic detachment. In terms of the former, her relationship with Peeta, a boy from the merchant class, and her attraction to the luxury of the Capitol make her question whether she might belong somewhere different. And as she grows more and more indignant as she observes the brutality of the Games, she is forced to make many ethical decisions. She ultimately shows that deep down, she is a caring and empathetic person who disdains causing suffering (even to the antagonistic Career tributes), as opposed to being only a stoic hunter. This theme is reflected in a running conflict of passion vs. reason.
The sustaining power of love
Love proves to be integral towards keeping Katniss alive. She survived the difficult times following her father's death because she had Prim to look after. Her love for her sister (and her mother, though less explicitly) is what helps her to stay strong as a provider for the family. Likewise, Peeta's act of kindness with the bread, which she credits as having given her strength in that period, was dictated by his love for her, though she didn't know that at the time.
Further, her growing love for Peeta in the arena continually helps her. The most obvious way is by encouraging Haymitch to send her sponsor gifts. Though she claims she expresses affection only for the sake