Gakusanjin is a mountain yōkai who used the Fuyōheki, a stone that masked the presence of yōki (yōkai energy), to pass unnoticed and avoid confrontations. The Fuyōheki pacified Gakusanjin, causing him to fall asleep, it also nullified his aura so that others could not sense him and therefore would not be forced into unnecessary fights.
Circa chapter 335 his Fuyōheki was stolen by Naraku, who entered Gakusanjin's body to claim it. Now perceptible, Gakusanjin was forced to leave his usual resting place and move in an attempt to recover the Fuyōheki. Gakusanjin was initially antagonistic towards Inuyasha and company during their first encounter, until Miroku pacified him and earned his trust by promising to retrieve the stone after for the Yokai after slaying Naraku . He learned of Naraku from InuYasha's group and lent them yōki-imbued crystals that they could use to detect the proximity of the Fuyōheki, since it was nearly impossible to locate the stone and itscurrent wielder through other means. Before he was found by InuYasha, he dropped many demon parasites which possess demons to which they are attached. Naraku sent Kagura to destroy the demon parasites to try to hide the fact that he had stolen the Fuyōheki.
He was shortly after killed by Goryomaru's children protegés, who did not bother to investigate whether he was a hostile yōkai or not. His stolen Fuyōheki was given by Naraku himself to Akago in order to mask him (and Naraku's heart inside him) completely from Naraku's enemies. It was later absorbed (along with Akago himself) by Mōryōmaru.
The stone did not prevent a newly trained InuYasha from sensing Mōryōmaru's yōketsu, until Mōryōmaru absorbed Naraku and his portion of the Shikon jewel, increasing both Mōryōmaru's powers and the stone's. During the same battle, the Fuyōheki was destroyed when Miroku tried to suck Naraku and Mōryōmaru (now merged into one being), or at least Akago (which would have destroyed Naraku as well) into his Kazaana. He failed, but the Kazaana's powers did manage to pull the stone away from them and into the Kazaana (along with vast quantities of shōki), thus destroying it.da:Skabelon:Animefigurstub