I'm pretty confused. That's a "Goblin" a what are his powers? I thought he was like the cu...

He doesn’t have powers. In Japan and China (maybe Korea?), they would refer to people who are different or strange as either ill omens, oni (demons) or goblins. The case here specifically is that in ancient times, children born as twins, especially identical twins, were not a good thing. And so to explain the existence of the second child, they blamed a “goblin”/concubine for putting a second child in the empress’s womb; thus making the second prince a “goblin”.

I hope that makes sense, it’s late and I’m tired, so it’s a little challenging to put my thoughts together