Belphegor

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

Since Belphegor is traditionally seen as having large horns, given his crest and power it is speculated that he is Nasu's contracted demon

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

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