Oni are demonic-like beings that have no care, sorrow, pity, any feelings for others, all they care about is their own selfish desires. They are foul creatures that go to planets and annihilate anything that breathes so they can take the planet for themselves, but they always end up destroying the planet, making it no longer inhabitable, so they move on looking for another habitable planet. And the one they want now is ours, Earth. So the Shinigami are doing everything they can to stop them from succeeding. Obviously by their appearance they are hideous, deformed looking creatures with filed fangs and broken wings. The Shinigami have yet to find out how they lost their wings and why. Also, the Oni have many scars, cause they love pain.