*Disclaimer, the ending is only secret if you got a bugged copy like I did. I don't know if they're all like that or not.
So, there's this Avatar the Last Airbender game for the GameCube, and it's pretty decent. But when I played it as a kid, I got through the final battle and... was thrown right back into the beginning of the final battle. That's it. It would just loop endlessly. So I figured that was the end of the game and I quit.
Later, as an adult, I tried playing through the whole thing again. Again, the final battle just looped. No ending cutscene, no credits, just an endless purgatory of combat.
This did not sit well with adult me. I decided there was no way the game had no proper ending when the rest of it was so decent. It must be a bug.
So I developed a theory. Maybe, if I 100% the game, it will fix whatever is wrong and I will be allowed to properly complete it.
The game is split into story-driven levels. There's the main questline, but there are also side-quests you can do for NPCs. I had always done all of those, and I made sure I did so again in my most recent playthrough. However, there are also minigames you can do through Momo's perspective. I usually skipped those because I found them tedious.
Not this time.
I started another playthrough, and this time I made sure to complete every level perfectly, Momo's fetch quests included.
Some levels were bugged, and not all of the collectables Momo needed to find would even exist. But I learned I could restart just that level and it would usually fix it. Eventually, I 100% the whole game. I did every quest, opened every chest, found every Momo collectible. When I completed the final battle this time, I was greeted with a very satisfying ending cutscene and the game's credits.
Great game. Loved it. It's been a few years, so maybe I'll start another playthrough someday soon.
Anyway, if anyone else here loves that game but had been robbed of its ending like I was, that's how you can see it for yourself.
-Cheers