The endings that I've found are:
Lose to Gustav: Get a very, very nice defeat scene. Genuinely great.
Cheese Gustav: Complete the gem fetch quest chain, and equip the reward to Anon to let him use his ultimate technique. Upon using said technique against Gustav, the battle is immediately interrupted by a cutscene with the ending. I'm pretty sure this is the joke ending that people mentioned, although it does have a pretty nice picture of Sana.
Legitimately Beat Gustav: I spent a while beating up Gustav's house staff to farm the gold to buy the equipment to do this. Anon with the best sword, shield, and armor, and Sana with the best ring. If you're going to do this, you're most likely going to have to do the same, and I would highly recommend going above and beyond to get bottles from the item shop as well.
You're going to have to get lucky on what attacks Gustav uses, but if you make sure to keep Karl alive and buff his Magic Attack via Sana it's possible. Or I guess you could use a cheat tool to set your stats high enough, if that's something you're willing/able to do.
Or I think Sana has a skill that can pause/interrupt/otherwise get an opponent to not use their turn? I haven't used it myself, but I think I remember seeing something like that in her skill list. You might be able to use that to get free set up/healing turns once you've killed Gustav's minions. Lmao, I might have made it much harder than it actually was.
Anyway, upon doing so you get a short little epilogue where Anon and Sana are married, and you get to see their new born child. Looking at the game files, I'm fairly certain who shows up here is determined based on Sana's corruption level, I personally got Kuma.
Edit: It seems I might be wrong about it being based on corruption. I thought it might be, because the pure ending seemed to have a similar picture, but other people are saying that who Sana gets knocked by is random, so I very well could be wrong.
Some things I ran into:
- The location where Anon is sent in the joke ending is labelled as 'error' in the little pop up that comes up on screen. Given that it's, well, the joke ending, I'm not entirely sure that's a bug, but I figured I'd mention it.
- Going into the recollection room and interacting with the 'Demon Lord's Castle Battle' scene thing prevents you from exiting it unless you watch one of the scenes.
- It doesn't look like the endings are available in the recollection room, as beating or losing to Gustav just puts you back at your last save. I imagine this is probably just a 'coming in the next update' thing.
- Oh, I don't know if this is a bug or an implication of Sana cheating on Anon, but while the girl at the inn in town four says that it costs 100 gold a night, it's actually free. Like I said, I don't know if this is actually supposed to cost gold, or if there's just the implication that Sana is busy working the cost of your room off while Anon is asleep.
Speaking of next update, something that would be kinda nice would be an 'unlock all' for the gallery. The main events already do so at the end of a chapter, but the roaming scenes remain locked, so a way to see those without having to spam entering/exiting a building for each town would be appreciated.
And yeah, as a couple other people have said, there are mystery items in the lairs of each of the three bosses. What do they do? It's a mystery! Guess we all just have to wait and find out.
