How did tetra become Zelda?
Tetras identity is revealed when the king of Hyrule fuses the 2 peices of the triforce
of wisdom together
Zelda does not appear in 2 games: Link's Awakening (DX) and Phantom Hourglass. In Phantom Hourglass, however Tetra is the alter ego of Princess Zelda, so you can still say that Princess Zelda did appear in this game, just in the form of Tetra. In Link's Awakening (DX) Zelda isn't even mentioned once.
According to the officials who created the Legend of Zelda series, and since Tetra is Zelda (sorry if you already didn't know), There are hints to such in the games. This is however assuming that All links and Zeldas are roughly the same in their reelings for one-another. Everything is implied, but not verified. Many people wish it was so, but there are so many other "Pairings" of Link and other people, that it would…
Each story changes but its basically like this: A hero named Link (who has an awesome green tunic) goes through a great adventure while collecting several items and weapons to save a princess (usually Princess Zelda, with the exception of Phantom Hourglass, where Link saves Tetra, however, Tetra is actually Princess Zelda) or the magical land of Hyrule from an evil villain, sometimes Ganon, Ganondorf, or sometimes another villan.