Jonas shows courage many times throughout the book, but in Chapter 21 he flees with Gabriel.
Jonas is brave. He demonstrates that consistently. When the Chief Elder is listing traits that Jonas needs to have, she says that The Giver reminded her that courage was required.
The Receiver himself was not able to describe it, only to remind us that you would be faced with it, that you would need immense courage. We cannot prepare you for that.
"But we feel certain that you are brave," she said to him. (Ch. 8)
On that day, Jonas thinks to himself that he does not feel brave. However, he is brave. He stands up, in front of the entire community, and tells them that indeed he does have this list of traits: intelligence, integrity, courage, wisdom, and the Capacity to See Beyond, and admits to them. That demonstrates some guts.
Throughout his training, Jonas does not back down to any challenge. He not only accepts painful memories, but asks for them when he sees The Giver writhing in pain.
"But I haven't suffered, Giver. Not really." Jonas smiled. "Oh, I remember the sunburn you gave me on the very first day. But that wasn't so terrible. What is it that makes you suffer so much? If you gave some of it to me, maybe your pain would be less." (Ch. 13)
In this way, Jonas shows that he is capable of compassion. He is learning from the memories, and acquiring the wisdom that goes along with them. As difficult as some of the information is, Jonas gains courage from them too. He starts to question his community. He begins to feel for the first time. From the memories, the inner strength Jonas already possesses is magnified into resolve.
When Jonas learns what release is, he has the knowledge of the memories to tell him what it means. He is horrified at what his father has done, and what his community has been doing. This is magnified when he learns that Gabriel is going to be released.
Jonas and The Giver had made a plan for Jonas to escape and return the memories to the community, in order to help the people learn to feel again. Unfortunately, Jonas has to rescue Gabriel, and he has to do it on his own.
He felt, surprisingly, no fear, nor any regret at leaving the community behind. But he felt a very deep sadness that he had left his closest friend behind. (Ch. 21)
As soon as Jonas learns of Gabe’s release, he goes into action. He is meticulous and surprisingly calm. He takes his father’s bicycle so he can carry Gabe, and some food. He escapes at night so no one will miss him. He cannot follow the plan they had, so he makes a new one. He knows that if he is caught, he will be released, but he does not care. All that matters is saving Gabe.
Jonas shows tremendous courage during the escape itself. He avoids search planes by using memories of the cold to keep his and the baby’s body temperature down. This shows good memory and quick thinking. He also uses the memories to sustain them as food gets low, or keep them warm as they run into snow. Through this, despite the ambiguous ending, he seems to keep them alive and get them rescued.
Jonas is the community's only hope. Although he says that he feels certain that they will condemn him if he is caught, the interesting thing is that we have no idea what is happening back in the community when he leaves. As soon as he crosses in Elsewhere, he must send the community reeling into chaos. Eventually they stop searching. Presumably they have bigger problems to deal with.