A bandit comrade of Tauemon, Ishimaru rebelled against their leader, Kaimaru's father, rejecting the group's chivalrous code as ineffective. He murdered the leader, cementing his ruthless philosophy of survival by any means, including kidnapping Myrthe and children for slavery.

Pursuing Tauemon and escaped captives to the Bridge of Heaven, Ishimaru confronted them until the goddess Sakuna intervened. His insult provoked her to kick him off the bridge into the Clouds of Creation below. The timeless clouds deposited him on Hinoe Island years before Sakuna and the humans arrived. There, he discovered the wounded evil god Omizuchi deep underground. Recognizing their shared hatred and destructive desires, Ishimaru forged a pact: healing Omizuchi in exchange for demonic power to command the island's hordes.

Using this power, Ishimaru systematically eliminated nearly the entire Ashigumo clan. He occupied a fortress once belonging to Sakuna's father, reorganizing the demons and fortifying it into Direhaze Keep to assert control over the island, preparing for Omizuchi's full awakening. Sakuna defeated him twice at the keep. During their second clash, Omizuchi awakened, triggering a volcanic eruption. Sakuna spared Ishimaru at Tauemon's request, allowing him to flee.

Stripped of his demonic powers after Omizuchi's influence waned, a desperate Ishimaru visited the humans' farm. He initially conversed with Tauemon but swiftly turned violent, attempting to take Kaimaru hostage. During the struggle, he injured Kaimaru before spectral forces intervened. Ashigumo and specters of his slain clan pursued Ishimaru, implying his death at their hands. Later, a corrupted manifestation of Ishimaru, controlled by the revived Homusubi Specter, was fought and defeated by Sakuna one final time.

His background reveals "Ishimaru" is a pseudonym meaning "son of a mason," adopted after his family was slaughtered during war, symbolizing his enduring quest for vengeance.