Prim is Katniss' younger sister who was selected in the reaping for the Seventy-fourth Hunger Games, but Katniss volunteered to serve as tribute instead. She works with her mother as a healer in District 12 to care for and cure the starving, sick, and wounded. Katniss is determined to protect Prim throughout the novel.
Just as she was during Katniss' first Hunger Games, Prim remains a strong motivating factor for Katniss during the Quell. Katniss has spent most of her life taking care of Prim by hunting food for her and volunteering to take her place in the Hunger Games. However, the announcement of the Quell and Gale's whipping allow for a role reversal between the two. In both instances, Prim is a major and welcome source of comfort, care, and love for Katniss. Katniss has spent years trying to be strong for Prim, but now it is Prim's turn to take care of her older sister. Just like Cinna, Prim is a contributing factor to Katniss' desire to overthrow Snow and save Peeta.