Hell is—well, hell .
For demons like Mikan Tsumiki, it was especially so.
The problem with Mikan is that she's meek. Most people don't even think she belongs here, as a demon in hell. I mean, what could she have even done in life that got her here? A passive, mouse of a person like her? That's the thing—they don't believe her capable of hate.
But Mikan knows she belongs here (even if no one else believes that). As explained to her, hell is a world based on your nature (whether ingrained or learned); and Mikan's nature was one of hate. Yes; Mikan hated the world, Mikan hated hell, Mikan hated the Heaven's that would reject her—she hated her bullies, she hated the bystanders, she hated everyone who had what she didn't; forgiveness.
Mikan was an unforgivable girl. It was her nature to hate. That is why, right here and now, she is in hell.
If she could, she would burn even this place down for it.
Junko Enoshima, on the other hand, had no fucking idea how she made it into heaven.
Apparently it was based on your nature? To which Junko could only say, uh, hey, hello? My nature is cruelty, I think there's a problem here? Like, what was God even on about? She definitely didn't belong here; and besides, it was so boring . Tormenting the other angels was fun (it seemed like they also had the same questions as for why Junko was let in), but even that got boring eventually. Apparently , it was Junko's nature to love; which she supposed was true—even things she should hate, Junko Enoshima loved, because they were absolutely, delectably despairing . But did that really qualify her for heaven, when her whole life she had been cruel?
Cruelty that came from kindness; came from wanting to set everyone free from their shackles of hope; despair given as a gift because of love—and evidently, that was good enough for heaven.
If she could, she would burn the whole place down, too—though that was only to show her appreciation, which, really, made it a completely different thing from Mikan.
But here's the thing: when these two oddballs meet, they change each other.
In Junko Enoshima, Mikan Tsumiki finds, for the first time, love. It wasn't in her nature to hate. Not really. All she did in life was hate, but that wasn't because that was truly her nature; it was because people only showed her hate, and so all she could do was mirror it back. But Junko Enoshima shows her love; forgiveness and tenderness and all things soft and wonderful. Junko Enoshima performs a miracle: she changes a demons supposed “nature.” Maybe that makes her more angelic than anyone; Mikan certainly thinks so.
In turn, Mikan teaches Junko Enoshima hate. Hate of all those who hurt her in the past, that hated her beloved in turn. Didn't they know what a wonderful peach they had? What a miracle Mikan Tsumiki was? In that, Mikan belongs in hell more than ever: she pulls an angel down from heaven and corrupts her with her darkness.
But Junko Enoshima didn't mind.
“I'll walk hell for you,” she tells her with shining, bright eyes. “I bet it's more interesting than heaven. Why don't we find Satan, and overthrow him? A throne for my queen, we can rule forever.”
And that sounds wonderful to Mikan Tsumiki, so she nods with a grin.
“Let's set it all on fire, beloved.”
This time, together.