Demon Constructor

The story follows an unfortunate High School student, who one day meets an old man ribbed by delinquents. The man asked the student what he desires as a favor for having helped him, but he didn’t believe the man and left. However, when he fell asleep at night, he wised he were stronger in order to take revenge from the delinquents. When he wake up in the morning, he saw his body changed and a demon appeared in front of the window…