While Layna means well and is right about trying to fix the world, one could call her out on setting Revya up from the start without even asking until AFTER the fusion. One could say she had it coming in the Demon Path.
One can also call how she deals with Danette. She sees a child suffering from the memory of her parent's death, so she seals the memory. Which also leaves her stupid to the point of brain damage. Virtuous-as-Layna clearly has no understanding of people, despite good intent.
Revya gets some of this thanks to the Demon Path. Particularly, was Revya's more vile personality in the Demon Path caused by him/her being Drunk on the Dark Side or was he/she Evil All Along?
Endorph's relationship with Euphoria. How much of his feelings toward her is actually toward Euphoria herself and how much of it is based on her similarity to his sister, Castile?
The first time you run into Heroman (no, not that Heroman), which is guaranteed to elicit a "what the fuck" the first time you see him. Even Gig is caught off guard by how silly he is. See Video Game Cruelty Potential for more.
Creepy Awesome: Demon Path Revya. Playing as him/her has you going down atrocity after atrocity with no sense of morality that put Kratos at his worst to shame, the only hints of redemption come if you lose Final Battle, and nothing you do is Played for Laughs. But as horrifying as that it is, it's undeniable that there's a degree of awesome playing as a Big Bad that ends with with a borderline 0% Approval Rating through their sheer depravity and still triumph of all that stands in your way.
Crosses the Line Twice: Much of the humor in Demon Path is definitely this. Case in point, Galahad, an extreme Butt-Monkey whose personality shifts from salesman to Prism Ranger among other things. He is going through these personality shifts out of sheer Sanity Slippage from all the atrocities you're committing.
Do Not Do This Cool Thing: Gig is by far the most foul-mouthed, ill-tempered, and just plain sociopathic character to ever come out of a Nippon Ichi game (Except maybe Demon Path Revya). He's also more badass than any five Disgaea characters put together; he's like what Overlord Zetta would be if he weren't stuck as a book.
Fridge Brilliance: If there was any doubt that Gesthal and Median are the same people, check out their moves: though rearranged, all of Median's attacks work the same as Gestahl: Hitting the row, column, or the entire party. And their skills are the same, just named differently; Median's being around his royalty, and Gesthal's being about revenge.
Fridge Horror: So, while playing the game, you have heard about Crimson Tears and seen them a bit often. Then at some point, you learn that by giving up their life, Danette's parents become those to save her. While that is fine by itself, when you add up that to the other stuff that happens... Did Thuris get his many, many Crimson Tears from his fanatical followers who would gladly give their lives? And it's mentioned that it cost a lot to have a Crimson Tear made. And Yesterwind, sells a lot of children and Orviska is their best buyer. And coincidentally, the real Dio has Crimson Tears as well. The more you think about it, the less you wonder why they have such a bad reputation.
Gestahl in the Demon Path; also the Secret CharacterMedian who has the same status and moveset as him.
Also, Hades' Despair rooms; their Outer Limit decor not only negates the penalty for having low stamina — one of the game's primary limiting mechanics — but causes you to gain a percentage to your stats equal to the percentage of stamina you've lost. Typical decors give you a 20% stat bonus in exchange for a hefty drawback; Hades' Despair gives you a bonus that can approach 100%, with additional advantages, letting you easily curbstomp otherwise equal groups. Oh, and it makes it so you die if your stamina ever reaches 0, but that's easy to avoid.
Ho Yay: Gig treats male Revya no different than female Revya. Right down to the soulmate business.
Also applies to Dannette, especially in her ending.
The male Levin ending also has shades of this. Revya essentially abandons his life, friends and family in favor of Walking the Earth with Levin to find the meaning of their respective existences.
Though she never reveals her past to anyone but Trish, Shauna's past sucked. Having her home hit with the game's equivalent to The Plague, villagers breaking in to her home, separated from her family and caretaker, forcing herself to fend for herself at a very young age. Of course, she'd probably kick your ass if you try to give her a hug, unless you're Trish.
Gig as he starts to regain his memories. He's still a Jerkass, but at he realizes that he was a good person wanted to help others whose memories were erased and he wrecked havoc on the world he was wanted to help, and enjoyed doing it.
The Demon Path as a whole for Revya, starting with killing Layna and everyone in his/her village.
Thuris the Scarlet Iago. He had no reason for this, he just wanted to kill people.
Kanan butchering the village of Pulkina (and Danette's parents) in the name of Thuris.
Drazil's plan. He tricked Median into killing Vigilance. But then he decides having souls flow into his world wasn't enough, so he sends Gig and the World Eaters to kill everything on Heaphness, and just to recap, this guy turn Gig into the Ax-Crazy nut he is at the start of the game and created Thuris.
One-Scene Wonder: When prompted to choose a title in the Demon Path, choose the second choice. Enter the voice of Laharl, which berates you for taking the title of an Overlord lightly and complains that he went through so much abuse for it, and disappears. Gig and Revya think they're hearing things, and no ever speaks of it again. The moment is one of the most famous in the game.
The Gideon class, which becomes available after you beat the game once. They've been compared to Majins in the Disgaea: Hour of Darkness for how broken they are.
The "Demon Force" Tactic, a 11th Hour Superpower received right before the final battle of the Normal Path, can greatly hasten the Level Grinding during "Inspection Room battles" when combined with the right decors.
Gestahl is all about smashing your entire party with massively damaging attacks. And his tactics are all about smashing your entire party and any others on a line behind them with massively damaging attacks, or turning his damage Up to Eleven. His decors are all about nerfing your parties so he can hit you for even more damage. It's easy to see why he's very difficult, even when he's entirely alone.
Unpopular Popular Character: Gig is disliked by most of the cast. Among fans, he's the most popular character in the game, many times being amongst the top 10 in Nippon Ichi's polls.
Viewer Gender Confusion: Kanan. An unusual case, everyone refers to him with female pronouns and he has the same class as all female Dracons. Basically, it's not just the viewers are confused about this one.
Tricia, especially in the Demon Path. She's more like a Iron Woobie in her ending.
Levin becomes one in his ending, where he's shown to have legitimate remorse over his actions across the game, but is too guilty to return home. He also has absolutely no guidance in life and wonders why he was brought back in the first place. The only companion he has is Revya, who he continually tries to get to stop following him, despite clearly wanting to keep them around.
Woolseyism: It's doubtful that the Take That! to John Romero during the Asagi fight was there in the Japanese version...