@shylarahno, rambles are good! So my notes..
Paragraph one: I didn't mean to infer the use of her as bait. More of something along the lines of needing her power to find something-- perhaps the Owner.
Paragraph two and three: this vague background is plenty. Details can be fleshed out later or in another discussion. What I like here is the ties binding the Owner and his lover to the Great Conflict. They're just players that have chosen to step away, but ultimately their part is not to fight. Rather their part is to aid those fighting. Ket would know them, at least from a past meeting. I'm thinking that early in the owner's life he was involved in a battle that was going south. Perhaps on his home world. Then, just before the decisive maneuver of the enemy, this guy -Ket- shows up from nowhere and begins killing demons left and right like they're nothing. Maybe the owner didn't see what happened, but he never forgot about that warrior who saved him from a death blow from a demon. Anyway the rest of that is good.
Paragraph four: I don't think he needs a name really. A man stepping away would surely give up the name that defined him before he decided to leave the battle field. He should definitely be suuuuuuper old though, old enough to have lived several lifetimes and seen many iterations of the Great Conflict and Ketsuoana's actions in them. Perhaps he has been watching Ket for a while now. I like your idea of him enabling Amuné to meet her destiny too. That's a good one.
Paragraph five: Okay, let's say that Kabuto gets his hands on another Seer of lesser power than Amuné, but enough to find her mind and invade it. What if he torments it and begins the corruption and this leads to what Tsitua needs to save her from. He's gonna do a thing that only he can do and that I will save for a surprise, that saves her, and this is what solidifies the bond. We can go from there with whatever we want to happen next. Perhaps a new rp will be born.
Paragraph six: Ketsuoana is both Angel and Devil. His mother was Zerulia of Light and Exoneration. His father was Deno, devil of destruction. He wouldn't interfere with things on a large scale. As you can see, so far he has only observed and influenced. Other than calming her magic that is. He is aware of the fates and usually doesn't act unless he is sure he is supposed to. It's a complicated thing. Point is, he won't always be there because the fates won't let him. This will allow mentioned events to transpire.