yay infodumps! tbh I like the sounds of all three.

For the investigation angle, Kadin in his original universe is actually extremely powerful, but he's unaware of this. Hell can't directly control him but they can lie to him. He believes he's just an incubus. Meanwhile the higher ups in Hell are rushing to find ways to control him before he becomes too powerful. His existence has been kept under wraps to prevent any power hungry lesser demons from figuring it out. So your guy could also sent out by group vying for power. Or the guys bureaucracy that have no idea and he gets sent like you said.

For Jun. oh my god. Kadin's mind is this patchwork of what'll feel like three different people but not quite. It's a total mess. Then there's Kadin's empathic abilities which could be neat to play off of.

Oh werewolf guy could be lots of fun too. I'm also good with playing off non-supernaturals, Kadin likes to live as a human as much as possible.

SO the problem is too many choices. Even without knowing more the manga character is up there in preference for antagonism alone. If I have to narrow it down it's him or wang.

Do you have a preference for location? I can generally come up with some bs reason Kadin is anywhere. As long as there is a "food" source.