The dragon is actually the dragon of reason and the face the our mc can see it proves that she’s the one who can help King Hadith free himself from a curse. In the future before Jill goes back in time King Hadith succumbs to the curse because he doesn’t find Jill before she marries the other prince.
Here are some spoilers from the novel whose link I’ve included below:
https://www.novelupdates.com/series/the-returnee-noble-lady-attacks-his-majesty-the-dragon-emperor/
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The dragon is actually the dragon of reason and the face the our mc can see it proves that she’s the one who can help King Hadith free himself from a curse. In the future before Jill goes back in time King Hadith succumbs to the curse because he doesn’t find Jill before she marries the other prince.