In Japanese folklore, a youkai is a monster, spirit, or demon who range from malevolent to mischievous to benevolent. They can take various shapes, some human- or animal like, or appear to be or possess inanimate objects. Some are shapeless, and others shape-shifters. They usually have spiritual or supernatural abilities.
Glossary: youkai
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youkai (妖怪)
Referenced from: Mirage 13, Mirage: Shinku no Hata wo Hirugaese, Mirage: Exaudi Nos
Bibilo: Yōkai (english)