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And I'm done.
If anyone wants to know how to unlock the true ending, here's how (putting spoilers in case you don't want to be spoiled):
1. defeat Aqua in the Port City stage. If you use the charge shot with Marine's weapon, you can defeat Aqua in 1 hit.
2. defeat Noel in the Yamato stage. If you use the charge shot with Aqua's weapon, you can also defeat Noel in 1 hit.
3. You'll unlock Noel and have to play as her in the Vesta stage. She has no double jump and no access to the other holomem's weapons that Flare already unlocked. Noel's only charge ability is making her own weapon hit harder.
4. defeat Miko but then the game doesn't end. There's two more boss battles. I won't spoil them for you, but I will say that Aqua's weapon is the most useful weapon to use against the very last boss.