Wondering if you have to worry about ending choices? Reached one you didn't like and are now bummed out? I gotchu, fam.
All Endings Guide
There are three endings, all told. Two are available by default, and one is unlockable. None of them are missable; you will be able to choose from the three from a menu in the final stage.
If you want to see all three, simply watch the (very short!) ending and credits, and you can return to the world map as if nothing had happened. The top of the mountain will appear on the map, and you'll be able to access the ending sequence immediately without replaying the final stage.
True Ending Unlock
In order to unlock the true ending, you must reach S Support with every party member. This will be simple enough to acccomplish in-game; battle with the party members to increase support (you can check your current level through the character status menu), talk with them at any wishfire, and do their personal quests (the destinations of which are visible on the world map, marked by an icon of their face).
Note that, at the time of writing, Ceryn's personal quest appears to be bugged. His face may not show up on the world map. Revisit the area after Drum Village. It should work as normal from there, launching a scene when you arrive.
S-support will change the final dialogue before the ending choice too. Be sure to talk to everyone on the mountain if you want to see this!
After entering the black hole at the top of the mountain, you will be faced with a spoiler-entity and a menu will appear after some dialogue.
By default, you will have two choices, corresponding to the Remain and Return endings. Both involve a short boss fight; I recommend spamming Viola's attack spell, which deals double damage. If you're interested in achievement completion, you can view both of these for an achievement each.
If you have unlocked the true ending, a third option will appear at the top of the menu. Select "Refuse" to continue to the true ending. A final boss fight with an upgraded final boss will ensue. After some dialogue, one more rhythm input awaits. Mash that X button and you'll be victorious! The final hidden achievement will be unlocked as well.