Mephistopheles, brother of Lucifer, and a fallen angel.

After defeating Satan together with Lucifer and Baphomet, Mephistopheles had a falling out with his two comrades.

The Celestials denied Lucifer and Mephistopheles their existence as Celestials, and thus the two became fallen angels.
Mephistopheles realized the dark side of the Celestials, and came to despise them for their arrogance. He wished to destroy the Celestials for their betrayal, but Lucifer, being immeasurably more wise due to his omniscience, did not agree with Mephistopheles.

Lucifer stated it's best to remain in the Infernal Realm to look after and protect the innocent Infernals, choosing to avoid provoking the Celestials in order to prevent a full-scale war.

Baphomet understood Lucifer's concerns, but couldn't quite sit idly by and letting the Celestials get away with their actions.
However, he grants Lucifer's ideals at least a chance since he helped him put an end to the Celestials's influence in the Infernal Realm.

Mephistopheles couldn't come to terms with his comrades' choices, and thus left the group.
Baphomet also left the group to prepare to chase his own ideals, but appointed himself as Lucifer's general and remained an alliance with him.

The Infernal Realm was thus split up in three domains.

Baphomet found a lot of popularity for liberating the Infernal Realm, and thus grew into a powerful demon. He founded his own kingdom, and was praised for being a good king among his followers.
His kingdom is situated in the east.

Lucifer remained in the center of the Infernal Realm, and introduced a democracy in his section of the Infernal Realm.
A lot of Infernals praised him for his good deeds, and he quickly got promoted to prime minister in the elections.

In the west, Mephistopheles ruled with an iron fist in his dictatorship.
Many feared him for his ruthless approach, but at the same time, criminal activity has been dwindling ever since he came to rule.

Mephistopheles still felt betrayed by his comrades and the Celestials. He grew bitter and ruthless.
His love for his brother Lucifer was now replaced with nothing but hatred.
At first, he wished to take Lucifer down, but he quickly realized that he was no match for Lucifer for as long as Baphomet was on his side.

Still, he had to take out his anger and frustration out, and finally did the unthinkable act.
He invaded the Celestial Realm, and massacred an entire Celestial army singlehandedly.
But that only made his bloodlust grow...

He felt like gods weren't fit to rule the mortals, and thus he attacked Solaris Diaruga, who actually had nothing to do with the evil Celestial Chaos Faction.
Despite Solaris's innocence, Mephistopheles successfully committed deicide against the God of Life, killing him.

Mephistopheles's victory granted him the power of life, granting him immortality.
However, the despair in his soul corrupted his power of life, effectively inverting his power of life into death instead.

Now he has attained the power of a god, Mephistopheles now feels confident enough to establish his Reason, his ideal world.

All he would need is a mortal vessel sunk into the depths of despair and another desperate person to summon Mephistopheles into the Mortal Realm with a physical form.

And this is the tale of the antagonists of the story.