Badass Grandpa: Father Salade. Just to give a hint, the first thing he does when it's time to get serious is singlehandedly take down a demon of the same species as one that you've fought twice now as a boss.
Berserk Button: If you value your life, do not mess with Croix's girl.
Bilingual Bonus: A lot of the characters, locations and spells are French words. In no particular order : La Pucelle (The Maiden, a Joan of Arc reference), Prière (Prayer), Croix (Cross), Culotte (Breeches, but a sideways Bastille Day reference as the revolutionaries called themselves the "Without Culottes"), Alouette (Sparrow), Eclair (Lightning, also a kind of pastry), Homard (Lobster), Salade (Lettuce), Papillon (Butterfly)... However, the bonus-est is probably Goddess Poitreen, meaning Breasts (although it's spelled "Poitrine" in actual French).
Bittersweet Ending: Although the world and everyone in it is saved, Alouette dies to save Croix, and despite having confessed love for each other, Croix and Prier part ways.
Black Cloak: Demons from the Church of the Divine Mother
Blessed with Suck: The Maiden of Light has awesome divine powers to defeat the Dark Prince yes, but she dies if she uses them all, and it pretty much takes all her power and then some to defeat him in the first place.
Particularly annoying in light of the fact that Mastiff (the North American localizers) fancied themselves to be the next Atlus, with all of the attention such a position carries...but sanitized the game because they were not prepared to handle attention and criticism from demographics unfamiliar to them. Even more irony is involved when NIS started an American branch about a year later, and had Mastiff not butted in, the damn game could have been released with no changes whatsoever. Not to mention the legal hurdles to port the PSP Updated Re-release.
Brought Down to Normal: Croix and Alouette. Alouette retains some of her power as the Maiden of Light, but it's nowhere near what she was tossing around during her fight with Croix. Croix was almost entirely de-powered, causing him to rely on his guns, at least until he unleashes his Superpowered Evil Side.
Combination Attack: Eight characters can appear on the field at once, and it is possible to have all of them attack at once if they're lined up adjacent to one another to attack the same group of enemies. Naturally, this also works in reverse.
Also, certain characters can grant another character unique Combination Attacks. For example, having Croix next to Prier gives Prier a special attack in which she beats the living hell out of the enemy, only stopping once Croix has his gun fully charged.
Defeat Means Friendship: Monsters must be "purified" to the point where they'll "join" you no matter what. After you defeat a pure-of-heart monster, it will join your team and can be summoned immediately, provided you have enough space to do so.
Becoming an Overlord lets you do the same thing to Demon class enemies.
Evil Costume Switch: Aside from the above example, this trope is played with, in that Croix switches to his normal outfit that you usually see him in when he became the Dark Prince. Played straight though, in that he more recently loses his Cool Shades when he unleashes his Superpowered Evil Side.
Fission Mailed: In the original game, the Demon Overlord "ending" happens when you kill enough Demon Overlords in Hell/Dark World. The original game mostly plays any ramifications for laughs. Later Nippon Ichi games and the Updated Re-releaseexpound on it.
If I Can't Have You: A villager in love with Angelique decides to sell her off to the Kingdom's "Witch Hunters" for execution because she was going to marry Croix.
Also, the story behind the first chapter has an entire castle getting murdered, including the count and his wife and son, because the count's brother couldn't accept that she loved the count and not him.
Improbable Weapon User: Culotte's method of attack is throwing various objects at the enemy - apples, lollipops, rocks, Mushroom monsters...just about anything he can find in those little pockets of his. Alouette often uses a Holy Book as a weapon.
Let's Get Dangerous: Father Salade is a perverted old man that has one of his female subordinates running around in a very inappropriate outfit, and doesn't seem too good at the whole priest thing. He can also kill demon lords alone and barehanded.
Level Grinding: It seems this is what brought about Nippon Ichi's love of this. It is possible to "reverse" the Gates which spawn monsters and enter Hell; or "The Dark World" as the westernization puts it; in itself a direct translation of the word Disgaea (hint, hint). Hell is a completely randomized dungeon filled to the brim with Demons, Demon Lords and Demon Overlords, stronger than any enemy in the main game.
Love Redeems: Prier's feelings for Croix help him break free from his dark side.
Luck-Based Mission: Going for the good ending in Chapter 10 are you? Better start praying to Poitreene that the computer's (random!) enemy movement and actions are agreeable, and that the sole mook type's paralyzes-inflicting special isn't used and doesn't work too often.
It's slightly better in Ragnarok due to the addition of the Re-Action system... but just slightly.
Mook Maker: The Dark Portals that appear in every level generate new monsters every so often if they're not purified.
Nipple and Dimed: A strange twist on the trope. Papillon's name was actually changed from her original Japanese name: Mamelon, which is French for 'nipple'. Poitreene got away with a deliberate mispelling. (Poitrine = Breast)
Relationship Values: Monsters that have joined your party have an individual "friendship" rating, which increases or decreases through "training" them. Harsh training rituals increase their stat growth, but decrease their friendship rating. If you bring this value too low, they'll leave the party, and there's no way to recover them. On the opposite end, if you decide to dote on them repeatedly to maximize their friendship level, they'll gain the ability to purify enemies.
Right-Hand Cat: Yattanya, Homard's first mate, is a rather literal example.
Schizo Tech: A somewhat regular fantasy setting is somehow filled with airships and walkie-talkies.
She's Back: Prier snaps out of her Heroic BSOD just in time for the battle with the newly-reborn Dark Prince, AKA Croix.
Shoot the Shaggy Dog: The Overlord Ending, in which Prier fights through the demonic hordes of the Dark World only to kill one too many demons and turn into a Demonic Overlord herself. This causes all of her friends and allies to leave her, alone and trapped in the Dark World with nothing but a sexynew makeover to show for it. Worse, this ending is canon: Overlord Prier makes cameos in every other Nippon Ichi game, and the Updated Re-release has an entire bonus story based upon what happens next.
Shout-Out: Eclair's character of a Princess with a Dark Alter-Ego is clearly homage to Princess Gradriel of Atlus's Princess Crown.
Stone Wall: Eclair, who has the party-best base defense and resistance combined (two others beat her in those stats separately, but are basically defenseless for the other) but low-end (for the front-row fighting characters) attack, and her speed is also low. Her dark half is a mixture of a Fragile Speedster and a Glass Cannon.
Take Your Time: Justified, Noir deliberately holds off on summoning Calamity until the party could arrive because Prier upset his ideologies, because she reminded Noir of his mother.
Tomboy and Girly Girl: Prier and Alouette, respectively and initially. Character Development sends both to opposite sides of the spectrum, Alouette becomes more and more willing to bend the rules and cut loose as the game progresses, and Prier reveals her inner warmth and compassionate side.
Theme Naming: Most character and location names are in French (Prier = "to pray", Alouette = "skylark", etc.) This includes some not so cute names (references to female anatomy and underclothing, for example).
There's also a hefty chunk that's based off food of all things.
Cerebus Retcon: The Demon Overlord "ending" was treated as a joke in the original game. This is lampshaded in the beginning of the new Overlord route, where even the characters treat the proclamation of Overlord Prier as a joke. Then the Creepy Child shows up...
Mind Screwdriver: The Demon Prier story explains what happened after the "Demon Overlord ending" in the original La Pucelle. In fact, it basically starts with that apparent "ending" and moves on from there.
Moveset Clone: While their special attacks are different, Laharl, Etna, Flonne, and the Prinnies have the exact same stats as Prier, Alouette, Culotte, and the Chocolats respectively.
Nonstandard Game Over: Father Salade kills Prier after she turns into a demon and attacks everyone in Church in confusion.
Bad Ending: Prier utterly fails to save Croix despite all her best efforts and he is destroyed- body and soul- by Noir (the local Smug Snake) whilst dying in Prier's arms. The following Unstoppable Rage causes Prier to be completely consumed by her Superpowered Evil Side.
Normal Ending: Same as above, however, a small part of Prier's "Light" seems to linger on in the real world as an individual entity.
Earn Your Happy Ending: Emphasis on "Earn" Prier becomes the "Maiden of Darkness" and purifies "Dark Prince" Croix such that his powers stay the same (Becoming the Hero of Darkness, no, not that one, with his free will intact; and Everyone Lives; even Alouette.
Noble Demon: While the original game averts this; Ragnarok makes it possible.
Same Content, Different Rating: The original game was rated Cero A, but Ragnarok was upped to Cero C. No explanation was given as to why. This is especially strange when you consider that Soul Nomad and it's infamous "Demon Path" is rated Cero A...
Unreliable Narrator: In Overlord route, Prier is succumbing to Darkness, so a lot of things in the real world may not quite be so. Like her seeing her friends as demons and demons as her friends. Whether she snaps out of it in time depends on a number of factors.