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 as a captive of the Solomon Company until he was released by Fuji.

History
Before Beelzebub III united the Demon World under his rule, Satan was the ruler of one of its countries.

Satan eventually fell into the hands of the Solomon Company sometime after until he was released by Fuji.

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.