what does author's imaginary boy in the mirror really represent in childhood? in adulthood?

The imaginary boy in the mirror represents everything that James cannot be or have during his childhood. He can be angry at the imaginary child and say what he will without fear of recrimination; the boy has to listen to him because he's trapped in the mirror, but even more importantly the boy cannot judge him for what he believes himself to be.

In adulthood, the imaginary boy represents the things that James has overcome in his life; he is a friend and a symbol of accomplishment.