When the world spins out of control, it is by your side that I find my steadiness. Somehow an adventurous prince and an independent herbalist discovered love together, found a steadiness side by side, and grew together.
Zen learned how to be more dependable and to become a much better prince, and Shirayuki learned to trust others, to let them in, and to grow alongside someone else. However, Shirayuki wasn't the only one to learn to trust and to let someone in. Zen had been learning this lesson slowly over the years, how to trust Mitsuhide in general and how to trust him after the Atri incident, how to trust Kiki and let her in, and eventually, Shirayuki.
How to trust someone who didn't know right away that he was a prince, and how to trust her to be okay with that reality without trying to take advantage of it in any way. That was a lot of trust, and that was a little scary at first, but also something that Zen felt a bit more willing to risk. Somehow, Shirayuki brought out his risk taking nature but also calmed him down all at once.
And like bluebells, there's a constancy here, a level of trust, and soul bearing. There's a steadiness and a humility to it. Zen isn't one to lord his title over others, and Shirayuki doesn't even address him by his title or think of it all that much. He's just Zen. Just like how she met him.
There's a loyalty between them that is steady and strong, even though their relationship isn't public knowledge. That's a risk, and yet that's a risk they both embrace and neither see it as an opportunity to betray the other's trust, but instead see it as an opportunity to build up their trust and make it even stronger.
They stay loyal, even if others might try to break that down.