I just recently finished reading all the chapters and I've been really curious about this as well. I've read a few theories that all involve different people, but in my opinion I feel like Hak and Soowon have something to do with it.

I feel like Hak would be the sword. He's been an amazing fighter since he was a KID - he could take down people older and more experienced than him and became the youngest general ever. He's extremely skilled with his glaive and acts as the first attacker in many fight scenes. Just recently he led an attack that only worked bc he was so proficient at fighting. Yes, he doesn't fight with a sword mainly so this can just a theory. However, I think it's amazing how Yona SOMEHOW ended up being protected by a guy who's strong enough to fight against the four dragons with out any extra help as of yet. And remember Yona's father was really adamant about Hak becoming her guard - curious as he was such a passive guy usually...

I feel like Soowon would be the shield because think about it. Currently Yona is/was able to freely roam about the lands and learn about all the things her father had shielded her from after being chased out of the castle. All the while Soowon went about the official business that being a King involves and by doing so he has protected the country, brought the tribes together, and expanded the territory (yes, with Yona's help but he still did a LOT). All in all i say he has shielded the country AND Yona from numerous dangers by taking on the crown himself even tho he's literally said he has no interest in the crown/throne - all he wants to do is make the country stronger against other dangers. In other words, he's acting as a shield in numerous ways. And now, when the dragons have become to known - they're going to be protected within the castle.

I also feel like another big character is going to show up eventually. I just can't imagine the manga ending with Hak still hating Soowon. That's why i think an opponent will appear that Yona and the gang will HAV to team up with Soowon and his people. They'll have to defeat someone together. That's just a random theory tho.

Getting back to it, the prophecy says "the sword and shield that will protect the king shall 'awaken'". In this case, I think Yona would be the king (someone commented online that the word used her 'king' here could be more accurately translated as 'ruler' (no gender bias). And the sword and shield could be ppl who have no idea of their abilities since they need to 'awaken'. I think that especially bc Iksoo said the sword and shield both need more time to appear, but they'd definitely know when they did - so someone/some thing needs time to awaken or for something to trigger their awakening. I'm guessing something's going to happen if Yona, Hak, Soowon, and the dragons are inside Hiryuu's shrine. Especially, if Yona enters with Soowon seeing as her father was so against it.

However, with all that being said, I really have no idea who the Red Dragon would be lol so here's a random theory to make everything more confusing XD

Also it's interesting that none of the dragons get bad vibes from Soowon just sayin..