As a result of his parents' disappearance, Naoyuki develops a type of avoidant personality disorder*. He isolates himself from others and has a hard time forging close bonds. Though he craves association, he fears rejection from his peers and finds it painful to open up to others.

A manifestation of the walls he builds around himself is his refusal to openly speak with others. This also makes him a target for severe bullying and builds him a bad reputation among his relatives.

*This is a real-life psychological disorder. I found out that Naoyuki's traits matched up with this, so I did some research and dug up this term.