Peeta Mellark was chosen as the male tribute from District 12 for the 75th Hunger Games. In the first book he was just the baker's son. Katniss had some connection to him because there was one time when he had given her bread when she really needed it. but that was before their first Hunger Games. When they were placed in the Hunger Games together they developed a bond with each other because they were fighting together. In Catching Fire he was determined to protect Katniss in the arena no matter what. So he told Katniss and Haymitch that if he wasn't picked he would volunteer as tribute. When the Hunger Games began he wasn't able to get off of his starting platform so Finnick and Katniss had to swim back to get him. He actually ended up dying in the Hunger Games from one of the traps that were set by the gamemakers. But Finnick knew if he had died then Katniss would give up so he gave Peeta CPR and brought him back to life.