Akuhei is Yamaorochi
There's a lot of belief he is, he has his same weapon Leviathan for one and both have similar traits.- Jossed. He's not Yamaorochi, Akuhei's power isn't the same level as Yamaorochi, and Yamaorochi was confirmed to have perished before the start of Control Sequence. Akuhei simply took his name.
Koyoto isEvil All Along
From what memories he does have he's had a dark life and he's likely only helping Nex to screw the government after his death because if he can't rule everything- no one can. The Voice Nex hears from the Azure when it's unsealed is the Black Beast
The Azure Nex gained was the same Azure Ragna obtained but was charged with chaotic energy if/when Ragna was turned into the Black Beast in the CTS timeline. The Beast can use the azure to communicate with Nex attempting to drive him further into his goal to destroy the world, and should that fail or not be happening quickly enough it has the seithr beings.