Basically, characters need to do something. Change something. Actually do something to the plot.
Katniss Everdeen, the main character, is introduced as a sixteen-year-old hard working, family loving, tough girl who is determined to do her best to stay alive and keep her sister safe. When her sister Prim is reaped for the Hunger Games, Katniss volunteers the second she remembers she can. In the beginning, Katniss is characterized mainly by a will to live and that dying love for her sister, even scorning Gale's and Peeta's rebellious speeches.