There are a total of three endings and below is a list of the different endings and the conditions in which you can experience each ending.
This is the regular ending and requires nothing special to attain. If you fail to meet the requirements of any of the other endings, this is the ending you are given. The game even pokes fun at you for getting this ending and encourages you to try again for a better one. The Chapter that determines whether you get this ending or not is Chapter 5 and 6. (See Good Ending for Details)
To unlock the “Good Ending” you need to acquire 5 items from events in Chapter 5 and Chapter 6. The five items required are, IF’s Notebook, Compa’s Syringe, Nep’s Pudding, Stuffed Doll and Peashy’s Drawing.
At the start of Chapter 5, view the events for IF and for Peashy and you will get IF’s Notebook and Compa’s Syringe. After you do one of the other town’s and beat the boss and clear the event, return to Planeptune for a new event there. At the end of that event you will obtain Nep’s Pudding. This is missable if you do not do this before completing all three nation events, so return as soon as you complete the first one.
At the start of Chapter 6, view Peashy’s event for Peashy’s Drawing and the remaining item, Stuffed Doll, is unlocked naturally by story progression.
Doing the above locks you in to the “Good Ending” away from the “Normal Ending” until the split happens towards the “True Ending”.
To get the “True Ending” you have to view events in 6 optional dungeons. Unlocking these dungeons can be a bit time consuming depending on how many optional dungeons you’ve done along the way and how far you’ve progressed in Stella’s Dungeon.
List of Optional Dungeons with Events
Once you unlock Extradimensional Space, you’ve reached the end of the chapter. You must view the event in each of the above listed dungeons in order to get the “True Ending”. If you are going for a Platinum or want to view both endings without doing multiple playthroughs, a good idea is to do 5 of the 6 events and make a second save. On the first save continue on with the game, and beat the game for the “Good Ending”. After you have viewed the ending, load the other save and complete the 6th event and proceed to complete the game. You will see a slightly different Chapter 10 along with the “True Ending” upon completing the game.
At this point, you are in the post-game and can do clean up on trophies you are missing and all the post-game story events.