Leviathan

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. Leviathan is currently residing in the Human World as a captive of the Solomon Company.

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

Leviathan eventually fell into the hands of the Solomon Company sometime after.