At the reaping Katniss shows selflessness (which is a trait heros usually posses) when she volunteers for Prim, because when she volunteers she only thinks about saving her sister.
Katniss not trying to kill Peeta in the games
Also at the reaping Katniss shows that she is very brave. Because when she shows up at the reaping and Prim is scared Katniss is more worried about Prim than she is herself.
Katniss having to go to the games
Katniss is not good at making friends, it is one of her weaknesses. So when she meets the other tributes she has trouble showing them who she is. Almost all heros have weaknesses.
The capital making Katniss into a rebel
Heros always want to help people, even though Peeta is technically Katniss's enemy in the games she still wants to find him and make sure he is okay. Haymitch said to Katniss "remember who the real enemy is". So Katniss unlike the other tributes is more concerned about the big picture.
Almost all heros have to take a big loss, Katniss's big loss was having o go to the games in the first place. But she (being a hero and all) had to volunteer for her sister. Because it was the right thing to do.
The capital made Katniss's life terrible. Her dad died because of an explosion in a mine that the capital made him work in. Her family is living in poverty and now she has to go to the hunger games. The capital made her hate them which is going to make her want to stop all the bad things they are doing.