The Devil Is a Part-Timer!, Vol. 20 (manga)

After the life-or-death battle on Ente Isla, Maou and company return to Japan to resume their daily lives, from the Devil King having his subordinates give him a haircut to the hero blowing off steam at the gym. But as the comfortable peace returns, there are still mysteries left unknown to Chiho. For instance—just how did the powerless demon boy known as Satan rise to become the Devil King in the first place?