Tetra IS Zelda...so regardless of her appearance, Zelda was present in her. She's the same person as when she appeared as Zelda. And as others have said, it's part of a series...Link, Zelda, an evil..an alternate appearance/name for the heroine doesn't change that.

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" I am the Hero of Time. No matter when, no matter where... for the sake of Hyrule and you, Princess Zelda...
I shall fight." - Link ( OoT manga )

Articles/Theories:
The Case for Kingship ~ Light and Shadow: The Tale of Two Princesses
Majora's Mask: Parallels in Love