Allie is the youngest son in the Caulfield's family. He died at the age of 11 because of leukemia. This character is described by Holden as the most intelligent and the nicest boy ever. Allie was red-haired, and he wrote poetry on his glove using a green color. These features make him special. He was even left-handed, and it gives readers a hint that Allie could be a genius.
When Holden found out about the brother's death, he was outraged and broke windows in the garage. It was a huge tragedy for the family, and this evoked lots of emotions in Holden's heart. He perceives Allie as his angel who observes him since death.
Allie Caulfield in the Essays