What are they?
I guess I would say they're akin to devils, but also to like ogres or goblins. They're selfish, predatory and dominating.
Where did they come from?
A hell dimension, or one of many. They don't necessarily all come from the same place, but their worlds are similar to each other. It's not important to the story that we get into more specifics about the hells they come from. They can come and go through hidden portals. They hide their portals jealously, but they can be found by humans on occasion. Humans cannot survive in the hell dimensions, and so crossing the portals is a death sentence.
Generally humanoid, but they don't necessarily have the same proportions or number of limbs. They're red, blue or black skinned, and have
masks instead of faces. If you pried one of their masks off, it would reveal a ravenous, endless maw, teeth all the way down the throat.
The Oni didn't appear suddenly or anything. In this world, they've basically always been around. They have only been in Burokun in large numbers for about a decade. Burokun is a poorer city, which is where Oni usually congregate.
Are they capable of emotion?
Definitely. Their emotions are tied mainly around themselves, though. They don't care about anything or anyone very easily. They
can feel affection, and if they decide they
do like someone, they are intensely and unreasoningly loyal to them. That's not terribly common, though.
Oni cannot die. Conversely, however, they also don't heal the way humans do. Or possibly at all, it's not clear to human observers. This is why Ronin mainly use swords. Guns don't work on a target that doesn't need any organs and can't bleed to death.
Oni will patch themselves up when possible, even to the point of sewing their own head back on. If they are too damaged to continue living among and terrorizing humanity, they will retreat to their Hell of origin, and usually aren't seen again within any human's lifetime. It is known, however, that Oni can and do return, thanks to comprehensive records taken over the centuries.
An Oni who loses their mask will rampage until rendered unable to move. It is other Oni who dispose of them, taking them from humans by agreement or by force. It is generally supposed that they are sent back to one of the hells.