I'm not sure if it's a translation issue but from what I can understand
- The King(Seme's younger brother) - is the previous ruler from the when the war was still going on (they're still a kingdom) and seme was still leading the war at that time.
- The Emperor (Seme) - Is the current ruler, after the war ended, he did a rebellion to the King and most likely declared the Kingdom to an Empire making him an Emperor.(I think he managed to make the Kingdom an Empire because he won the war)
- Idk why they're still calling the previous ruler "King" though so most likely a translation issue.

I don't understand the difference between the king and emperor... It makes me so confused cause ain't they both the ruler?