Specifically in the novel The Hunger Games, and not including the other two books in the series, Katniss Everdeen's character changes through the influence of her experiences in the Capitol and in the arena of the Hunger Games. From the very beginning of the novel, she reveals herself as being a character of great heart and compassion for the ones she loves, like when she volunteers to take Prim's place at the reaping.
As the novel progresses, she becomes even more focused on surviving for Prim's sake, but the challenges of the Games force her choose between survival and her own humanity. From Rue's death to the horror of the muttations, Katniss has begun to despise the Capitol, but also has the strength to challenge their regime. By the end of the novel, the reader sees a Katniss who has matured even more and has finally realized the true dangerous nature of the Capitol.
She turned from someone who was poor to someone that ended up winning the hunger games.
Katniss starts out as a strong independent woman and by the end of the series she is depending on Peeta. She looses her independence and becomes weak to both Peeta and Gale.
Through the whole series she kind of.. undevelopes. She's not a Buffy, she's not a Bella. She limps across the finish line when we're used to seing main characters sprinting. She lives the rest of her life with just Peeta and her kids, because the real Katniss isnt the one that stories will be told about. Someone that finds life almost unbearable? No, she has no place in their stories. Which is really the only reason I think she "undevelopes". Ofcourse you could argue that through dialogue and plot Susanne Collins developed the character traits and personality of Katniss Everdeen. You could say she realizes a lot of things about life, and because of her misfortune she's grown. Either way, I guess