Yōkai

Yōkai (妖怪; Literally meaning "demon", "spirit" or "monster") are a spiritually aware race of "parasites" that possess a certain spiritual race and feeds off of their energy, giving in exchange the ability to increase their spiritual power.

Overview
The Yōkai race are spiritually aware parasites that lodge themselves into another race's body, mostly to Shinigami. In exchange for their feeding off of their energy, they allow them to use their powers or increase their existent ones. The only known Yōkai yet is Sora Shirakuni, who possesses latent Shinigami powers inherited from his father. His powers are also weakened because, even though he is a Shinigami, he was sent to the Human World so he would be free of harm, which, with time made his powers fade as he didn't put them to use.

It seems that these monsters are more common to appear in those that have hidden and dormant powers, since they can feed off that big energy reserve without causing much harm and because, if the host had bigger spiritual energy, they would have a hard time housing themselves, as the energy itself would neglect them.

Appearance
Every Yōkai possesses a different appearance, some more humanoid than others, but some looking like complete monsters. Some are even spirits that are completely made out of energy.

Known Yōkai

 * Anki (闇鬼, Dark Demon): Lodged onto Mark Vernier. His special ability is to increase the host's physical prowess and spiritual power.

Trivia

 * Yōkai are Japanese folklore creatures, similar to demons, that possess strange abilities, some good and some bad.