Youmu

Youmu (妖夢, lit., "catastrophic illusion") are strange creatures that roam the human world. They come in all sorts of shapes and forms, and their origins are currently a mystery. Youmu have been known to possess (to kill) humans and are extremely hostile. For this reason, these creatures are constantly targeted by Spirit World Warriors. They are also called "dreamshades" in some translations.

Characteristics
Youmu vastly differ in appearance, with forms similar to that of animals or non-living objects. The similarity among all youmu is their tendency to manifest an ore-like stone that contains the powers of that youmu when slain. These stones are sought after due to the value appraisers give to them.

Humans normally do not possess the ability to see youmu, which can only be seen by Spirit World Warriors and other youmu. However, interactions between youmu and humans are a constant source of concern amongst Spirit World Warriors due to a youmu's tendency to possess a human and act towards them with hostility. In the light novels, the issue of discrimination against youmu extends to half-youmu and Spirit World Warriors with youmu-like abilities (an example being the Kuriyama clan, to which Mirai Kuriyama belongs).

Half-Youmu
Half-Youmu (半妖, han-you, where "you" refers to "youmu") is the term referring to the offspring of a human and a Youmu. Due to the rarity of Half-Youmu, they are often met with hostility by Spirit World Warriors, who know little of their true nature. The only known Half-Youmu in the present is Akihito Kanbara, who is currently being put under the surveillance of the Nase family of Spirit World Warriors, in order to know more about Half-Youmu and to eliminate him immediately when he proves to becomes a threat to others.