There's a glow about her, a lot of hard work leading here, just all of that combined within Shirayuki somehow made her even more beautiful than Zen's ever known her to be. Just the way she works herself sometimes seemingly to death, the way she still pauses to take care of people when she sees they need something, just all of who she is.
Zen has never known anyone like her, and he knows somehow that she is the only one of her, that she's the only one he will ever get to know that is like this. He knows somehow, that very remarkable quality of hers is uniquely hers. If someone has a similar quality, they wear it differently and would be beautiful in a different way.
Shirayuki glows on mission, glows like phostyrias, glows beautifully. She fights for every victory and somehow makes it there too. She is strong and resilient, beautifully dedicated. She keeps on going.
He's never known of an awe that spills over, that bubbles up, like the awe he has for her. Just the way even when she doesn't surprise him, but is herself the way he knows her to be, he witnesses that bubbling up of awe, despite anticipating her choices.
She's incredible, and Zen's more in love with this amazing woman than he ever anticipated he would be too.