As you know, Noppera-bo looks just like a human being, but he has no eyes, nose or mouth. Noppera-bo does not harm people.
However, it would be very frightening to find him just behind you in the dark as you walk along the street alone at night and happen to look back.

Nuppeppo
A priest decided to stay at a ruined temple on the way to his destination as it had become dark.
At midnight he was aroused by the sound of a big noise and could smell something terrible - like the smell of a dead body. He opened the fusuma (papered sliding door) of the room, and saw a chunk of dead flesh walking in the corridor...!
Thus, Nuppeppo appears at midnight in ruined temples or near graveyards. It might be the body of a dead person walking by itself.