When Peeta Mellark is selected as the tribute for District 12, all we really know about him is that he's a baker's son, a little bit emotional (3.47) – and that Katniss really wishes he hadn't been the one chosen as her co-tribute (2.23). Over the course of the novel, though, we learn that he played a large part in helping Katniss's family survive after her father's death. For this, Katniss feels deeply indebted to him. Peeta is also totally and completely in love with Katniss Everdeen.
Peeta's character serves, at times, as a contrast to Katniss's. Whereas she is a provider and a survivor, Peeta is just the opposite: he's not much of an outdoorsman, is in touch with his soft side, and comes from a world very different from Katniss's. (His family, while they end up eating stale bread, never goes hungry: they are of the more privileged merchant class.) As such, Peeta's character helps develop many of the novel's major themes: love, hope, class, and identity.
Let's explore them:
Peeta's been in love with Katniss ever since he first saw her back in grade school and heard her singing that catchy valley song. He's the one who instigated the whole romance plot by confessing his feelings for Katniss on camera. As for Peeta, he truly believes in the part he is acting. Peeta is way into all of the kissing and cuddling and love talk. His father, we also find out, had it bad for Katniss's mother. A love of Everdeen women is generational in Peeta's family. His ongoing affections make him a symbol of the hopeless romantic – a role he plays exceedingly well both on and off camera. His feelings for Katniss capture the audience's heart (and some of the readers' as well), attesting to the power of romance and its attendant narratives.
While Katniss means everything to Peeta, let's not forget that he also means something very important to her. Not exactly love, no. Still, we learn from Katniss that in her eyes Peeta symbolizes hope. He is, after all, the one who helped save her family from starvation by giving her loaves of bread when she was a child (2.48). Peeta's kindness probably saved the lives of Katniss and her family. This means that, while Katniss is grateful to Peeta, she also feels an uncomfortable sense of obligation to him for his kindness and generosity.
Katniss associates Peeta with dandelions: the flower that she and Prim used as food after her father died (2.58). Katniss mentions that, again, this all has to do with hope. But how else might Peeta be like a dandelion to Katniss?
Peeta's family lives in District 12, but they are not coal miners. Rather, Peeta's family – his sweet father and unpalatable mother – are bakers. His parents' occupation puts Peeta one rung up above Katniss on the class ladder (though still far below the citizens of the Capitol). This might seem to be a rather small difference, but Peeta's class perspective very much shapes the way he sees the world.
It's not that Peeta's soft exactly, and he's proved he's not a coward. But there are things you don't question too much, I guess, when your home always smells like baking bread, whereas Gale questions everything. (22.57)
Peeta has, perhaps, a different view of political matters than Katniss and Gale. What else might he see differently because of his class privilege?
While Peeta's relationship to Katniss is very important for the novel, his character is also significant in that he is one of the tributes who considers what his identity means in the scope of the Hunger Games. As he tells Katniss as the Games approach, he doesn't just want to be a pawn in the Capitol's game. He wants to "die as himself" (10.70). For Peeta, it is important that the Capitol knows that they don't own him.
We should also remember, though, that much of Peeta's reflection on himself has to do with the fact that, unlike Katniss, he is not of the survivalist mindset. Instead, Peeta contemplates the Big Questions. This all makes Katniss feel kind of small:
While I've been ruminating on the availability of trees, Peeta has been struggling with how to maintain his identity. His purity of self. (10.71)
Katniss may feel guilty about her focus on survival strategies for the big day, but we could also always argue that, since Peeta has no duties or obligations to his self-sufficient family (the survival of others doesn't depend on his own), he has the time and resources for self-reflection.