Among Ghosts

Luke is 12 years old, and he has never been to school, never celebrated birthdays, and can barely read. Unlike his two older brothers. He lives in the attic in secret from everyone, while his brothers can play with friends and wander wherever they want. He has never dared to appear in open spaces, even if there was not a soul around. What is wrong? Why do his parents look away when he just wants to pull back the curtains and look at the road?