How is Bruno from The Boy in the Striped Pajamas a hero?

In The Boy in the Striped Pajamas, Bruno is a hero in how he sacrifices himself for another person.
Boyne's "fable" makes Bruno a hero.  The way Bruno honors his word and displays selflessness is heroic. When Shmuel needs his help to find his father, Bruno is not only a good friend but also quite heroic when he says "I wouldn't let you down."  This heroism is underscored when Bruno realizes that life on the other side of the fence is nowhere near what he thought it to be.   He is open about his desire to return home.  However, when Shmuel reminds Bruno of his promise to help find his father, Bruno's heroism is evident: "Bruno thought about it.  He had promised his friend that and he wasn't the sort to go back on a promise, especially when it was the last time they were going to see each other."  While his confidence is lessened because of what he sees, Bruno does not back down from honoring something more than his own needs.
Bruno's heroism continues when the boys are ordered to march into the gas chamber.  There is pure terror in the moment before "the door at the front was suddenly closed and a loud metallic sound rang."   However, Bruno "did something quite out of character" in holding Shmuel's "tiny hand" and saying that he is his best friend for life. Like a hero, Bruno rises above the pandemonium and sadness around him.  His heroism compels him to remind his friend they are going to experience this together.  Boyne concludes the chapter by showing that despite the chaos in the gas chamber, "Bruno was still holding Shmuel's hand in his own and nothing in the world would have persuaded him to let it go."  Bruno does not leave his friend in a time of need. He sacrifices his own comfort and overcomes his own fears to reassure Shmuel of their friendship.  These actions make Bruno a hero.  His innocence is a reminder that heroes do not have to be extraordinary.  The ability to care and to live by one's word is heroism.