who's in the wall

Mu Chen has a secret, he has a special "friend".Contact him in a way that ordinary people can't imagine. "Listen" to his "speak" and chat with him.He will comfort him when he is sad, he will chat with him when he is bored in class, and he will tell him the correct answer when he encounters difficulties in exams and homework.This "friend" is only known to him, because he is inside the wall...