It could be a former tribute who now works as a guide for new tributes. This character has experienced the games first - hand and can offer unique insights. They can ride along with the new tributes during their pre - game preparations, sharing their own survival tactics and warning them about the Capitol's tricks. Their main goal could be to try and give the new tributes a better chance at survival, which makes for an interesting character arc.
Perhaps an undercover rebel. They could pretend to be a Capitol citizen interested in the Hunger Games and get to ride along with various Capitol personnel. This would give them a chance to gather intelligence about the Capitol's operations and weaknesses. They could also interact with the tributes in a way that subtly encourages them to resist the Capitol, all while trying to avoid being discovered as a rebel.