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. He is one of the strongest demons encountered so far, able to overpower Behemoth, and apparently split Hell in two with his battle with the Great Demon Lord.

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

He also fought with Beelzebub IV's father, the Great Demon Lord, and presumably lost.

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

Return to Ishiyama High School Arc
After petrifying most of the students at Ishiyama  and everything in a 10km radius, he and Fuji slowly close in on the survivors and the newly arrived people, namely Oga, Furuichi, and Hilda. He petrifies Behemoth's 34 Pillar Division and Behemoth himself. He and Fuji break through a barrier erected by Himekawa, and petrify Kunieda, seemingly along with Oga. Oga summons Beelzebub IV just in time and his third eye opens. His left arm is blasted off and fuses with Fuji, gaining wings and horns

. They spar with Oga and Beelzebub, seemingly winning, but they eventually start losing to Super Saiyan Oga. He releases the petrification spell to gain more power to aim one final blast at Oga. Apparently the blast would be so powerful everything in a 10km radius would turn to ash. In the end, he is defeated by the Zebul Finisher, and they defuse.

Petrification
He is able to turn targets into stone statues with by touching them with black mist. He's able to manipulate the mist at will so it can chase targets. This ability affects demons and humans alike but costs a lot of power. When he stopped the power, targets were returned to normal.

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.