Fridge Brilliance: Why is Atsuro your strongest physical fighter initially, even though he's a scrawny hacker? He hacked his COMP!
Why does Belial say at the end of Day 6 that humans are more qualified for the throne of Bel than demons? Because humans invented something that can allow humanity to go toe-to-toe with demons.
Why does God and his Angels abandon the earth on Yuzu's 8th Day? Because as Lucifer said only he will never abandon us.
Why does Naoya suffer from Redemption Demotion in Atsuro's route? Because he simply can't be arsed to fight Babel for you. Why should he exert himself for a cause he's not particularly invested in?
Lucifer, I read your book! Up to this point, his motivations have generally be described as For the Evulz and It Amused Me, but look closely at what Nebiros and Belzaboul (on Naoya's route, at least) say upon their defeat—both of them say something to the effect of "my master will be pleased." Combining that, on either Gin or Atsuro's route (I forget which, but I want to say Gin's), Loki will imply that Lucifer will visit the human realm (and we know he does), despite his apparent lack of interest of the title of Bel. And finally, on Naoya's route (at least), Belberith hints at wanting to overthrow Lucifer once he becomes King of Bel. While this may seem like a stretch, it seems to me that Abel's involvement in the conflict was planned by Lucifer, especially if what he says about being the main character's father are true: Worst case scenario, it'll at least be fun to watch. However, if Abel wins the throne, he had someone that got rid of a potential usurper. Bonus points if he decides to rebel against God (anyone else remember Nocturne's True Demon Ending?). And hey, even if Abel decides to be a lapdog of God's, all Lucier needs to do is wait for him to screw up. he stands to gain something regardless of the outcome.
The MC is a reincarnation of Abel. In other words he's a Bel.
Fridge Horror: When you first revisit Keisuke about 2 hours after he leaves your party, you'll notice that he's gained some experience and new skills, including some most likely cracked from human adversaries. And it turns out that given his later behavior, it's very likely he gained these by killing some people. So when Atsuro tells you not to let Keisuke become a murderer, you're probably already too late...
More a meta-example rather than in-game. Let's just say you're a newly-wed couple. Then you finally have a child. Turns out the child is a reincarnated killer with all the memories of four thousand years living in the world INTACT. Granted, you won't even know who exactly the child is, but seeing that he's more likely to be anti-socialyet earning perfect scores in every test would be, well, unsettling...
Speaking of which, Naoya spent most of his "childhood" being raised with the protagonist after his parents died. Considering how focused he is on his "brother," it makes you wonder whether he had anything to do with that...