Oni

Oni (鬼), commonly translated as ogres or trolls, are a unique species of Ayakashi able to form contract partnerships with human demon tamers. Individual oni may vary widely in overall appearance, but they have basically humanoid bodies, and nearly all have clawed hands and feet, wild hair, a number of fangs protruding from their mouth, and two horns growing from their head. Skin color is also varied, with known examples in the series including green and white. Oni range in size from human-sized to several times larger than humans, with the largest standing taller than a multi-story mansion. Classical oni are sometimes depicted as wearing animal-skin loincloths and carrying spiked clubs called kanabō, and this at least true for some that appear in the series.

Oni also seem to vary in intelligence. On average, they possess human-level intelligence and are capable of speech. Others are admittedly limited in this area, and rely on simple speech or growls to communicate. Some do not seem to speak at all, but at the very least are able to communicate with human demon tamers through some means.

In terms of physical strength, oni tend to be far stronger than humans. Even those of comparable size can perform inhuman feats such as lifting boulders larger than themselves with a single hand, or knocking a person out merely by flicking them with a finger. Larger oni are far stronger, and even those considered weak by their own standards possess overwhelming strength when compared to humans. Some oni also have additional unique powers, such as ice manipulation, flight, or rapid tail movement.

Oni are also known for their ability to form contracts with human demon tamers. The contract is formed when the human gives the oni a name, as well as a charmed tool, which acts as a visible symbol of their bond and a means of nonverbal communication. Highly experienced demon tamers, however, appear able to communicate with large numbers of oni without using such a tool.