Youkai (妖怪, yōkai) is a term mostly associated with Japanese folk creatures and ghosts. The term can also apply broadly to anything that is a supernatural being or legend, from any culture. Because there are thousands of stories of magical beings around the world, the different types of youkai are endless. Fairies,vampires, oni, and nekomata can all be found in Gensokyo. Alternatively, it is a common Japanese belief that after a certain amount of time, ordinary objects and animals gain life and personality, becoming a kind of youkai as well. In some cases even a normal human can become a youkai under the right conditions.
Gensokyo youkai tend to live within the wilderness, far away from humansettlements. Normal youkai seem to behave like wild animals, attacking humans within their territory on sight. Exceptional youkai are less common, but they often enjoy speaking to humans, and are much stronger than an ordinary Gensokyo human, one on one. These youkai are almost always humanoid in appearance, and can be quite unpredictable; friendly one moment, hostile the next.
Many, though not all, youkai are known to eat humans on occasion, if not on a regular basis. Many youkai are also stronger and live longer than an average human, with lifespans often reaching into hundreds of years (or possibly more in some cases). One common trait often seen among youkai is a weakness to various faith based and spiritual attacks, charms, wards, etc. In Gensokyo it's therefore natural for the resident shrine maiden to become involved whenever there's trouble being caused by a youkai.
However, despite the danger certain youkai present, humans and youkai can and do live together. Some, such as Tewi Inaba, are even helpful to humans. Deciding if a youkai is harmful to humans or not must be done on a case by case basis.
It seems that youkai, like fairies, may not be able to die permanently; Hieda no Akyu, Reimu Hakurei and others regularly discuss 'extermination' of youkai as if it were only a temporary measure. While this may be merely a quirk of language, it seems like Gensokyo's humans would rather do away with many youkai completely, were they able, it is mostly known that there are more Youkai's then there are Humans in Gensokyo currently, though Gensokyo is getting more and more Human's everyday, it is noted that in the next few years that Human's will have more...but it's just a myth. though it does seem real as most people are saying these days.
- Example (normal): most enemies that appear during a non-boss encounter are weak youkai/fairies.
- Example (exceptional): Rumia, Yukari Yakumo, Yuuka Kazami, Hong Meiling, Orange, Tokiko, Ichirin Kumoi, Rinnosuke Morichika (Half), Yumeko (?)