Doodle, the younger brother of the narrator called "Brother" has a variety of things that make him unique. First of all, Doodle (so-named because of his habit of crawling backwards, like a "doodle bug") has a number of physical problems that set him apart from people who are "normal." He cannot walk normally, so Brother devises a wagon to pull Doodle around until Brother eventually teaches his disabled sibling to walk. But these things, the physical disabilities and challenges, are not what make Doodle "unique."
Doodle proves himself to be unique in a number of more subtle ways, not the least of which is defying the expectation that he would live at all. So sure were his parents that he would not live to see his first birthday, Doodle's father had a coffin made for his tiny, malformed son. When Doodle defies the life-sentence his doctors had given him, the coffin was stored away. In an act of cruelty that Brother would later come to regret, he shows Doodle the coffin one day:
One day I took him up to the barn loft and showed him his casket, telling him now we all had believed he would die. It was covered with a film of Paris green sprinkled to kill the rats, and screech owls had built a nest inside it.
Doodle studied the mahogany box for a long time, then said, "It's not mine."
In addition to his will to live, Doodle is also unique because he sees things that others typically take for granted, like the beauty of the natural word. Brother takes Doodle to a favorite spot he knows named "Old Woman Swamp." While most people may not think of a swamp as beautiful, to Doodle, it is. When they arrive, to Brother's surprise, Doodle begins to cry:
"For heaven's sake, what's the matter?" I asked, annoyed.
"It's so pretty," he said. "So pretty, pretty, pretty."
The Scarlet Ibis has much in common with this young boy. Both are rare and unique. When the rare bird crashes into a tree beside the boys' home, no one knows what it is and their father orders Brother to go find the "bird book" so they may look it up:
"It's a scarlet ibis," he said, pointing to a picture. "It lives in the tropics--South America to Florida. A storm must have brought it here."
Unfortunately, this rare bird does not live:
"..the wings were uncoordinated, and amid much flapping and a spray of flying feathers, it tumbled down, bumping through the limbs of the bleeding tree and landing at our feet with a thud. Its long, graceful neck jerked twice into an S, then straightened out, and the bird was still."
The end of the story is terribly sad. Brother has always lived in a place of tension in regard to Doodle: he loves him but he is also ashamed of him and pushes him hard to be something he is not. He tries, in essence, to make something special and unique ordinary and colorless. It is not until Doodle passes away that Brother understands what a brief and special person he had had in his sibling.