Prim is Katniss's twelve-year-old sister, of whom she is fiercely protective. As Katniss says, "I protect Prim in every way I can, but I'm powerless against the reaping" (1.63). This turns out to be not entirely true. Prim was originally chosen as District 12's tribute during the annual reapings, but Katniss made the ultimate sacrifice for her family and volunteered to take her sister's place.
Prim serves as a contrast to her big sister. She is a more conventionally feminine character, for starters. Unlike the tough, no-nonsense Katniss, Prim is quite sweet, cooks, and loves animals (including Buttercup, the family cat). As Katniss says, "People deal with me, but they are genuinely fond of Prim" (3.23).
Prim, we should note, is also a nurturing, skilled healer. For example, she owns a sweet little goat named Lady, a formerly wounded animal Katniss rescued from being butchered many years ago (Chapter 20). When Katniss brought the hurt goat home, Prim was able to bring it back from the edge of death by "grinding up herbs and coaxing brews down the animal's throat" (20.65).
Katniss thinks she could never possess Prim's healing powers, but she does manage to bring Peeta back from the edge of death by doing much the same that Prim did with Lady. Maybe the two sisters aren't quite as different as Katniss thinks.