The Devil Is a Part-Timer!, Vol. 20 (Manga) - Satoshi Wagahara

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?