What events lead to Katniss's character development in the Hunger Games?

When Katniss begins the hunger games, she is young, but already more mature than most her age. She provides food for her mother and sister by hunting and gathering and supplies by trading at the Hob. She also takes the tesserae so her family can have enough food to last then through the year.
Volunteering for her sister at the reaping is the first step in her development. In District 12, being a tribute is considered a death sentence. Katniss accepts this in an effort to save her sister’s life. She is elevated in the eyes of the community when she does this.
Another event that influences Katniss's development is hearing Peeta's desire not to be a piece in the Capitol's game. She doesn’t understand this at first. Later she realizes she doesn’t want to be controlled either. She buries Rue in flowers and sings to her when she dies, knowing all of Panem will see it. This is how she starts to rebel.
When she pulls out the berries at the end and threatens to eat them, which would end her life, it shows how much she has changed. She starts out as a girl who just wants to get back to supporting her family and ends up a symbol for the oppressed in the entire country.