Sadao Maou / Satan Jacob is the main protagonist of the series. He was once the Demon King who declared war on Ente Isla in an attempt to annex the four continents. However, due to the actions of the Hero Emilia Justina and her companions, he was forced to retreat to Earth, causing him to lose almost all of his powers.
Satan was well-known throughout Ente Isla as the great overlord of his people, his name synonymous with the terror and cruelty his army brought to those unfortunate enough to fall victim. He allowed his minions the right to perform any heinous acts they deemed fit, showing immense cruelty through the slaughter of many innocent humans in his conquest. By contrast, on Earth Sadao Maou is shown to be a diligent and proactive employee, instead applying his strong drive and determination to better himself in his new life. Forced to spend time living as a human and interacting with those he previously considered to be lesser beings, he has since developed a kindhearted and cheerful disposition making him popular among both his colleagues and neighbors. He now strives to climb the corporate ladder at MgRonald's.
Though he is not the legendary Satan spoken of in the Ente Islan Bible, Satan Jacob was given the name at birth, as it has since become an extremely common name among demons. While he once belonged to a lowly goblin-class tribe, Satan's family and comrades were slaughtered in a conflict between demons, the enemy having left him behind to die following the carnage. While lying motionless on the ground awaiting his eventual demise, he was found and rescued by a mysterious angel, who chose to care for the child and ensure his survival. The stranger claimed she had never seen a demon crying before and nursed him back to health, sharing with him many stories and her knowledge of humankind. Through her stories, he became instilled with a desire to unite his own kind the same way that humans had done in the past.
(Source: Hataraku Maou-sama Wikia, edited)