Cause he called him Apel
he just knew
He just knew. Karas said it himself, his laugh, words, mannerism, and the care that he felt from the fairy not knowing he is the 'Your Highness'. Then, to add the cherry on top, when he mumbled 'Apel', Abel instinctively responded to it. It wasn't just a sleeptalking thingy but rather his response to what he heard, unconciously. Habits die hard. Plus, the fact that Karas knew that the fairy doesn't know who he is yet but he called him 'Your Highness' while smiling fondly is a shoot shoot.
No i mean abel himself how did he know that it was his highness?
He doesn’t he just instantly responded to being called Apel because that’s what his highness would call him.
Now how the heck did he know that it was him?