Youkai (妖怪) are spiritual or demonic creatures in Japanese mythology, and constitute the majority of the enemies (and allies) in Inuyasha.

In the Inuyasha Universe, there are at least three different types of youkai.

The weakest youkai are flying snake-like demons. They consist of a grotesque head – often human or animal-like – and a long trailing tail. They can sometimes manifest as small globes of light. While frightening, they generally are not very threatening, as they lack appendages with which to strike. They are relatively unintelligent and pose little more than a distraction to the Inuyasha-tachi, except in cases where they amass in groups of dozens or hundreds.

A stronger breed of youkai are those which take on the appearance of animals, such as centipedes, praying mantis, lizards, weasels, bears, and so forth. Regardless of the natural size of whatever creature they appear to be, these youkai can be as large as a man or even ten times a man’s height. These youkai are much stronger, and defeating a single one may trequire the full attentions of one or all of the Inuyasha-tachi. They are known to possess religious icons and homes. They are also quite crafty, and often ambush humans, use disguises, taunt them, etc.

As Sango once told Kohaku, the most dangerous youkai are the ones that take on the forms of humans. These are especially cunning youkai, and usually very strong. Most of these youkai are still animal youkai despite their human-like appearance, and some can transform into their animal form to take advantage of increased size, strength, and range of attacks.

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Youki, jaki, and shouki mean almost the same thing in Japanese: an evil aura, or feeling of unpleasantness. However, Takahashi makes the following distinctions in Inuyasha:

Youki is the aura that youkai give off, flowing from their bodies like a river or wind. It can only be sensed by those with spiritual training, or a heightened sense of youkai-smell. Thus, Miroku and Inuyasha can sense youki while Kagome and Sango usually cannot. Inuyasha visualizes youki like flowing water when he uses his kaze no kizu attack.

Jaki is the name for an evil aura surrounding a cursed place or object. Jaki can be sensed much more easily than youki, and manifests as a deep sense of foreboding. A sufficiently strong jaki in a place makes a visit very uncomfortable. A strong jaki on an object would corrupt its possessor, destroying his will utterly. For example, the sword of the Oni teeth, Toukijin, held the jaki of the Oni, possessing the swordsmith who forged it and filling him with the need to seek the Oni’s revenge. It would appear, due to his spiritual training and her priestess-like abilities, Miroku and Kagome can “see” jaki better than the others, such as when affixed to an object. However, most times, all in the group can sense a jaki spread over a large area, such as a town.

Shouki is a thick, noxious vapor Naraku uses to scatter, redirect, or kill his enemies. In small quantities it can overtake most humans. In large concentrations, it is visible as thick violet vapor, lethal to humans and youkai alike.