Youkai Physiology

Youkai are primarily spiritual beings rather than physical ones. As such they are more vulnerable to faith-based attacks (such as charms, wards, etc.) than to physical injuries and ailments; it's said a youkai can even recover from being cut into pieces, if the damage has insufficient "meaning" behind it. Some youkai, such as magicians, have bodies more similar to humans and may lack this ability.

Related to this, most youkai keep their promises once given. While a human who loses a Spell Card duel can immediately challenge the winner to another, youkai rarely do so.

Many youkai are stronger and longer-lived than humans, with lifespans reaching into hundreds of years or more. Marisa claims that Rinnosuke Morichika, who is only half-youkai, has not visibly aged since she was a child. Unlike fairies, however, they are not immortal.

All youkai have an innate connection to the Moon - according to Yukari Yakumo it was the moon's dim light which gave birth to the youkai by causing humanity to fear and create legends about them, while Kaguya Houraisan claims that youkai and magic were created by Lunarians in order to regulate the earth's "filth".

Over time many of Gensokyo's youkai have come to take on human traits and vice versa. Youkai with human-like bodies and/or personalities have become far more common, even among normally monstrous species. However, even the most human-like youkai would still be considered strange by human standards.