They are the strongest enemies of humans in Gensokyo.

Their livelihood is kidnapping humans, and with their high levels of physical and magical strength, they are the strongest beings in Gensokyo.

In antiquity, they lived on Youkai Mountain and, employing the tengu as servants, built up an oni society(*1).

From then on at regular fixed intervals, they would kidnap humans, and thus threaten the livelihood of the villages.

However, since several hundred years ago, the number of oni has decreased little by little, and from the time the great barrier was built, they no longer show themselves anywhere in Gensokyo.

Just as few humans were trying to exterminate them, they were none too pleased with the way in which the world of Gensokyo had become so carefree, and they simply went to a different world. No one knows where this world is, and since then, contact of any kind with the oni has ceased(*2).

Oni can have a wide variety of physiques, from huge builds all the way down to short and petite.

But all of them have horns, and some also have fangs.

A few have arms longer than humans with slouched postures.

Their personalities can range from cheerful to ferocious, and they can suddenly become belligerent with those they dislike. However, once they recognize someone as a friend, then their feelings will become intensely warm, and the idea of betraying or double-crossing you would become inconceivable.

What they detest the most are gossiping, cowardice, malice or unfairness.

Always honoring their promises, they can think of no other way to behave than to be fair and square.

There are no youkai who are as forthright as the oni.

Also, their emotional and mental discipline is considerably strong, and very few oni are not well-versed in the principles of Zen.

Also, they love alcohol like no other being, and can consume vast quantities. If you were to become drunk with them and then lie down to sleep, no matter what happens you won't wake up.

1: It is said that the present society of tengu is built upon the vestiges of the original oni society.

2: In Gensokyo, we call where they went the Land of the Oni. Possibly Hell.

Since times fairly long ago, humans have been tormented by the oni.

Occasionally descending from the mountain, kidnapping humans and making off with sake, the oni threatened their day-to-day life.

And because the humans couldn't stay silent and do nothing, an oni extermination specialist was trained or summoned at fixed intervals to go exterminate the oni.

When that happened, marvelous tools and implements were taken from the oni society after they have been beaten.

Oni and humans, through human kidnappings and oni exterminations, might have possibly built up a relationship of mutual trust.

Because the oni have disappeared from Gensokyo, at the present time, there are no oni extermination specialists.

Because oni are different from regular youkai, the means of extermination and the tools necessary are particular and specialized.

The sorcery to exterminate oni have already been lost by humans, thus it would appear that meeting an oni nowadays would be too much for anyone to handle.

In reality, the way in which things like the Tsuina ceremonies or other oni-repelling rituals are conducted are already inaccurate.

But then, the chances of meeting an oni are already mostly non-existent so they should be no problems, but...