If you had to point to one moment where Kingdom Hearts started getting a bit complicated, it’d be the entirety of Kingdom Hearts: Birth by Sleep. It’s also where Kingdom Hearts starts to resemble Star Wars in a few ways — prequels, ancient catastrophic warfare, Jedi…er, Keyblade Masters, and so on— but unlike Star Wars, this prequel is actually necessary, and no spot is more important than Aqua’s finale.
The build-up to her confrontation with Xehanort takes the focus on bonds and friendship to a completely different level. Sora might not get to spend much time with Kairi after finding her again, and Riku’s got his own problems to sort out, but they’re all still together; they work through their problems and come together when it matters.
Not so for Aqua. Not only does she have to sacrifice herself at one point to rescue Ventus and hopefully find some way of saving Terra, but it’s completely hopeless. Ventus slips into a seemingly endless slumber, while Terra loses himself completely to Xehanort, becoming his vessel and, by extension, the source of conflict in the mainline games.
Friendship didn’t save the day here. Even if it provided the means by which hope could be restored in the future, it still left one main character broken in the end, and that’s something you don’t see much in Kingdom Hearts.