They are neither plants nor animals. They differ from other forms of life such as the micro-organisms and the fungi. Instead they resemble the primeval body of life and are generally known as "Mushi". Their existence and appearance are unknown to many and only a limited number of humans are aware of them. Ginko is a "Mushi-shi" who travels around to investigate and find out more about the "Mushi". In the process, he also lends a helping hand to people who face problems with supernatural occurances which may be related to the "Mushi".
(Source: Anime News Network)