Yeah agia doesn't seem to be "ready" to fullfill that role you're right, love your theory, because i think about the same XD
I feel Ken right place is by Agia side and not the vampire side. just a feeling, but I feel the vampire will try to harm Agia again.
I feel Agia is partly the god that take his power from Aman diractly without knowing it of course XD
It's seems to be a very ancient myth, so I agree with everything except Agia being the one deceived (If I'm wrong and it's a recent myth, then Agia being guilty would explain the bad rumor surrounding him)
So for my theory... Lol
Hmmm so there are three variables?
1 human of the night
1 human of the moon
Vampires.
My guess is that Ken is the human moon? If i remember correctly mr. Vampire refer to Ken as his Sun? My guess is he's like a sun/hope when he can only thrive in the darkness?
If mr. Vampire is really the first vampire ever. Then he's the one who stole the moon's life. My theory is that when he is still a deceitful monster. He was also drawn to the moon's kindness(beauty? or immortality?), He was actually jealous not of the moon's power but because moon care for humans. But then his greed unexpectedly took away moon's life and he's actually suffering by only surviving on human's blood at night (i mean how ironic right if that's the case, a monster who hate humans now need their blood to survive; only thriving at the night where the MOON is just above him, engulfed by DARKNESS/NIGHT ). And no matter what he is still drawn to the moon, even the human moon, Ken. (lol if this is the case, i pity you, just come to me vampire hottie)
And i think Agia is actually the person who was deceived leading to the death of moon.
Human of the night i think is gonna be someone else. Or maybe Agia will get an upgrade to become the human of the night? I just think he's not really it right now.