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 Demon World 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.

Powers and Abilities
Immense Demonic Power: As a member of the Seven Deadly Sins and thus being one of the rulers of the Demon World before its unification, Satan is an immensely powerful demon and is easily one of the powerful demons and characters in the series. Laymia stated that Satan at full power was able to easily overpower Behemoth and Hilda stated that when Satan fought against the Great Demon Lord many years ago, their battle had split the Demon World in two. Another example of his power is that he was able to easily defeat most if not all of the 34th Pillar Division on his own after petrifying most of the human residents of the town and after they had formed a contract with Furuichi is use their full powers in the human world. After fusing with Fuji they were able to compete against Oga and Baby Beel's Super Saiyan Switch even after Satan expended his power petrifying the demons and humans of the town before being overpowered and needed recall all of the power he used to petrify them to stand a much better chance against Oga. Petrification: Satan has the unique ability to petrify both humans and demons alike into stone statues by touching them with a black mist. He's able to manipulate the mist at will so it can chase targets but costs a lot of power to use. When he stopped the power, targets were returned to normal.
 * Energy Slash: Satan was able to create a powerful slash that can decimate a good part of an area by swiping his arm in a slashing motion.
 * Energy Blasts: When fused with Fuji, they are able to cast powerful demon energy blasts that can decimate most of an area even after Satan's power was diminished from petrifying the humans and demons alike. After recalling all of his power, they were able to create an energy blast that was able to compete with Oga's Zebul Finisher to the point where he needed to place the King's Crest on everyone aligned to them to absorb their power to overpower Satan's power.

Corrosion: Satan also possesses a unique ability to corrode any objects he touches like a metal fence of Ishiyama High which turns the fence into black mist.

Immense Speed: Satan is fast enough to petrify all of the 34th Pillar Division on his own within seconds before most of them could react. When he is excited about his battle against Baby Beel, Satan was to move faster and made quick attacks that Oga was forced to defend against them instead of dodging. After fusing with Fuji, the two become much faster that Oga was forced to convert Baby Beel's electricity into his own body to match them.

Immense Endurance: Satan is very resilient in battle where he was able to continue battling against Oga and Baby Beel after expending a good deal of his power petrifying everyone in the town and showed no pain at all when his left arm was blown off by Oga. After fusing with Fuji, they were durable enough to suffer a great deal of damage from Oga's Super Saiyan Switch with diminished power before needing to recall them.

Immense Strength: Satan is strong enough to flick aside Laymia's energy blasts with his fingertips alone.

Enhanced Agility: Satan is agile enough to launch many attacks while in mid-air.

Flight: After fusing with Fuji their fused form gains a pair of black wings like a Fallen Angel that enables to fly at high-speed.

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.