A youkai is a supernatural being from Japanese folklore. In Higanbana no Saku Yori ni, several youkai are gathered at the school because of its massive size. They like to feed on the souls of humans and play mischevious tricks. In some cases they will even attack other youkai.
There is a hierachy system in place at the school with seven (later eight) seats being given to the top ranking youkai. Those youkai are: