Growing up without friends, both makes one super grateful to have friends and a little unsure of how friendship really works. Shin-Ah's not used to navigating the socialness of friendship, like how does having a friend really work?
And then, there's Zeno, who is not much for social conventions, and after living a long time, may have some hidden uncertainties about friendship, but who jumps in with a smile and cheer most of the time, ready with a hug or a joke or laughter or a smile. He makes awkwardness disappear simply by embracing it.
You don't have to be perfect around Zeno, and you certainly don't need to have everything figured out. Zeno plus you is all you really need. And Shin-Ah isn't sure he's doing the whole friendship thing right, but when he's included by Zeno simply being Zeno it feels nice. Like he's taken under the older Dragon Warrior's wing somehow, like he's going to be protected and taught and that friendship will be unconditional with him.
And then, there's a moment when Zeno determines that it is time to fight, that it's okay if Shin-Ah uses his eyes or his sword on him, because it buys time, and Zeno will be okay.
But Shin-Ah may not know much about friendship, but hurting his best friend is not an option in his eyes. It would be better to protect him, to team up with him, and fight enemies together. And just know Zeno's in pain or seeing him hurt his friend would be unbearable.
Shin-Ah figures he might not be the best friend, but he's stubborn when it comes to keeping those he loves safe. And having a friend after not having one certainly teaches you a lot, a lot about friendship, and a lot about appreciation.
So, he's not going to let his friend get hurt, not by his hand, not ever by his hand. They'll be fine, and they will get through this together. They are friends, best friends, and that is enough reassurance to last a lifetime.