Symbolism, Imagery, Allegory
When you were a little whippersnapper did you have a pacifier, a baby blanket, or a special stuffed animal that you just refused to let go of? (We did. Her name was Annie and she was beautiful even when her ears fell off from too much snuggling.)
Five-year-old Gerry is no different: his trusty sidekick is named Blankie. As you may have guessed, using your Sherlock-ian powers of deduction, Blankie is a blanket. (It'd probably be weird if that was the name Gerry chose for his teddy bear.)
Obviously, a security blanket is a symbol of childhood—but Blankie doubles as a symbol of the end of childhood, too. When Gerry gets upset, he often puts Blankie over his head. The thing is, Blankie doesn't block all his problems out—a secret he confides in his brothers:
Gerry took a long breath and looked at Dylan and me. "Did you know I can see you right through Blankie?" he asked. (10.48)
Gerry isn't a baby, or even a toddler, anymore; he's five years old, so it makes sense that Blankie's security isn't operating at a hundred percent capacity. Gerry's at a sort of in-between age, and Blankie is the bridge from early childhood to being a more mature kid.