Link was for sure shocked learning Zelda was Hylia... but imagine Zelda's shock when the first time she sees her normally sleepy, silly and smiley best friend after being separated...
He's beaten and bruised, completely covered in blood, most of which definitely isn't his own. His eyebags have gotten deeper and darker. Despite his exhaustion, his eyes are shot wide open. His face once had soft curves, but now it has sharp edges, and his whole face is gaunt. He looks almost completely rabid. It's like staring at a different person entirely.
Except, when she fully meets his gaze, he melts right back into the boy she'd stay up all night giggling with. He completely cheeses, and his eyes sparkle like he's about to burst into tears. She can't help but do the same. But Impa's arm coming between them is a painful reminder that they've got things to do, and so little time.
