Satan

is a Demon who is a former ruler of one of the Demon World's countries prior to its unification, thereby being a member of the Seven Deadly Sins. Satan is currently residing in the Human World, before as a captive of the Solomon Company and at the present as contracted demon with Fuji.

History
Before Beelzebub III united the Demon World under his rule, Satan was the ruler of one of its countries. Eventually went into a slumber until he was awakend by Iris and then fell in the hands of the Solomon Company sometime after.

Trivia

 * Satan, in the Abrahamic religions, is the personification of evil and temptation, often described as an angel who fell out of favor with God.
 * According to Binsfeld's classification of demons Satan's sin is wrath.