No problem, here's some answers. 

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. 

What do they look like?

Generally humanoid, but they don't necessarily have the same proportions or number of limbs. They're red, blue or black skinned, and have [url=http://imgur.com/gallery/Z878SZV]masks[/url] 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.

Backstory?

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. The Ronin have only been around for about five years, but the show is only in its second season. 

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 [i]can[/i] feel affection, and if they decide they [i]do[/i] like someone, they are intensely and unreasoningly loyal to them. That's not terribly common, though. 

Why do they take over neighborhoods instead of just eating everybody?

Put simply, it's easier. They did, once, but this current arrangement has been working for them pretty well for a long time.