Greetings and welcome to the guide to every ending, in this guide all endings will be shown so beware of spoilers. I will attempt to discribe how to get every single ending so even rookies to the series will understand what to do.

Getting some of the endings are needed to unlock some of the music in the Music Shop (Red Skull in the castle) but otherwise its just for achivements sake.

Alot of the endings depend on ally kills made, seeing how many ally kills you have made can be seen at the Netherworld Records (the Male Healer in the castle).

Edit: I have gotten a few more questions about what the game counts as ally kills so I did a bit of research:
-Killing ally with any kind of geo chain does not count as getting an ally kill
-Throwing Prinnies and making them explode DOES count as getting an ally kill
-Killing specialists in the Item World does not count as getting an ally kill
-Killing a clone made from one of your characters does not count as an ally kill
-Having a character lift another character and having the lifter die does not count as an ally kill (Even if the lifted character is an ally)

It should be noted that after every ending your game will end, asking you to start a new cycle. After a new cycle your story progress (and ally kill count for that cycle) will be reset, meaning you have to start over again giving you a chance for a new ending, (Non-Story) Characters and items stay though.

All these endings are gotten after killing the final boss.

-Good Ending
Getting this ending requires to kill the final boss without getting any ally kills, meaning you if you have killed any of your allies and saved the game this ending is now unobtainable.
Unlocks Flower of Happiness in Music Shop.

-Normal Ending
Getting this ending requires atleast one ally kill made, this one speaks for itself just beat the game while having one or more allies killed, simple.
Unlocks Flower of Happiness in Music Shop.

-Bad Ending 1
Getting this ending requires atleast one ally kill and passing 100 bills by force. Passing bills by force means suggesting a topic at the Dark Assembly (accessed by talking to Pleinair in the castle) and it getting rejected, after that selecting persuade by force and killing every opponent. I recommend saving before attemting to persuade by force as doing this can sometimes be difficult, especially at lower levels. As far as I know there is no way of tracking the amount of bills passed by force although I might have missed something.
Unlocks Tragic Marionette in Music Shop? (Needs to be confirmed)

-Bad Ending 2
Getting this ending requires atleast one ally kill and killed an Item God in a Legendary item.
Killing an Item God requires getting a Legendary item (an item with a golden name and golden crown above its stats) and killing the Item God inside the Item World of that item. The rarity of the item determines how far you can go in a item, Legendary items being 100 floors deep. The Item God will appear on the 100th floor, killing it is required for this ending.
Unlocks Tragic Marionette in Music Shop.

These endings are gotten in the middle of a playthough.

-Flonne Tragedy Ending
Getting this ending requires getting 50 or more ally kills before the end of chapter 3. After defeating the boss you will be asked to kill him, awnsering yes will award this ending. Selecting no will continue the story as normal.
Unlocks Tragic Marionette in Music Shop.

-Etna Heroine Ending
Getting this ending requires getting 100 or more ally kills before the end of chapter 5. After defeating the boss you will be asked to kill him, awnsering yes will award this ending. Selecting no will continue the story as normal.
Unlocks Etna Boogie in Music Shop.

-Human Conqueror Ending
Getting this ending requires getting the map "Human World" and finishing all the levels there. Getting the Human World maps requires having a rank 7 on the Dark Assembly with a character and suggesting the topic Human World. The maps here can be pretty difficult for a first playthough as the highest enemies fought here are around level 200.
Unlocks Lord Laharl's Hymn in Music Shop.

-Mid-Boss Ending
Getting this ending requires you to lose to the reoccuring boss Vyres better known as "Mid-Boss". Simply getting all 10 characters killed in battle against him will earn the ending.
Unlocks Ah, My Magnificent Life in Music Shop.

These endings can only gotten in Etna Mode.

-Normal Etna Mode Ending
Getting this ending requires you to just beat the game in Etna Mode, really, thats all.

-Beauty Tyrant Etna
Getting this ending requires you to beat Tyrant Overlord Baal in his map in Etna Mode. This ending will most likely be the most difficut to get. Getting to Baal requires you to beat 2 other areas: The Alternate Netherworld (rank 8 Dark Assembly) and Beauty Castle (Rank 9 Dark Assembly) in any order. Baal is EXTREMELY DIFFICULT he is considered the ultimate challenge in the game and should only be attempted with well trained, well geared, max leveled characters.

This was my first guide so if there is anything I can improve, tell me. Other than that this information gotten from a other version of the game (Disgaea: Afternoon of Darkness PSP) but as this is a remake the information should still be completely viable. It would be highly appriciated to tell me so.

Good Luck getting all the endings!