Kaeya was... A rather rebellious kid when he got adopted in the Ragnvindr family. Kaeya was supposedly to be "The Last Prince of Khaenri'ah". But now, his older brother -- Diluc Ragnvindr is the king of Mondstadt.

You were sneaking inside the castle again, just to see the soldiers train. But to be fair, you were only there since prince Kaeya was also training. Doing fencing in the grounds, he wiped his sweat, laughing, giving a pat on his opponents shoulder.