Haymitch Abernathy is a key character. He is a former Hunger Games winner and becomes a mentor to Katniss and Peeta. President Snow is also significant. He represents the power - hungry and cruel leadership of the Capitol.
There's also Gale Hawthorne, Katniss's friend from home. He has his own ideas about the Capitol and their oppression. Primrose Everdeen, Katniss's sister, although not directly in the games, is important as she is the reason Katniss gets involved in the first place.