To me, there are so many heroes in this book.
I would say that Johnny and Pony are heroes. This is partly because they pulled the kids out of the fire at the church. But Johnny is a hero for helping to "save" Pony. And Pony is a hero for snapping out of his depression and managing to retain his enthusiasm and ambition.
I would say Darry is a hero. He has given up his ambitions so that he can help Soda and Pony have a better life.
I would say Cherry is a hero because she is the one who first starts to bridge the divide between the Socs and the Greasers.
So, to me, heroism is A) overcoming obstacles in your way and/or B) helping others in an important way if it is hard for you to do. All of the people I mentioned qualify in my opinion.