Youkai (妖怪) 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. Their appearance can be the same as humans, but can have additional features such as wings. Youkais do live in the Hiiro system and the small area called Gensokyo on Earth.
A youkai usually have super natural powers, such as the ice fairy Cirno, to the explosion-manipulating vampire Flandre Scarlet, and the border manipulating youkai Yukari Yakumo. Some youkais may have no super natural powers, but just bullet hell cards.
Usually, when humans come from planet Earth in the Sol system, they usually pick half-human and half-youkai. One example is Furan, who is both. Sometimes humans pick what they are, however.