What has changed between Tetra fearlessly ninja-kicking Ganondorf in the face, and then when she's in the basement going "Go help Hyrule. I will wait here"? That's not depth, that's two diametrically opposite characters. All that has happened is she is now wearing a dress and was told she's a princess. And yet she goes from a beacon of confidence to a delicate little flower. Everything from the way she holds herself to the way she's written indicates a distinctly different character, all because of a wardrobe change...
This moment
is literally the only hint you have in the entire back half of the game that she's even remotely the same character as this:
And you can say this somehow doesn't make her one-dimensional, except the whole time she does anything other than be captured or occasionally use some vaguely useful magic spell (only when it's convenient for you, of course) is when you're still under the impression she's not Zelda. Like, if you're older than the age of 10, you've probably already figured it out. But even still, on the narrative's terms, her
becoming Zelda pretty much strips Tetra's character of any of its energy.