In "The Scarlet Ibis," which of Doodle's characteristics enable him to learn to walk?

In addition to being persistent, Doodle is innocent when it comes to the harshness of this world.  He cannot stand to process the negative aspects of reality such as the coffin that Brother shows him (which was intended for Doodle) or the abandoned dead scarlet ibis outside their house.  To combat these shocking truths of life, Doodle dreams, and his dreaming personality enables him to envision himself walking, running, doing all the things that Brother does.  In his dreams, he also sees himself living in an idyllic swamp kingdom, where he flies high above all the troubles and concerns of the world.

Doodle also possesses selflessness, which causes him to want to please Brother and his parents more than anything else.  He endures physical pain, Brother's condescension, and more so that he can make Brother and the rest of his family proud of him.