Arthur Oshihara is a high school student practicing amateur exorcism with his assistant, Mita. Firmly convinced that all supernatural entities are inherently dangerous, he identifies Kirara's ghost as an evil spirit demanding banishment. This conviction drives him to actively attempt exorcism rituals against her spectral presence.

He is motivated by a protective instinct toward the living, particularly Konpei and the younger Kirara, whom he sees as potential victims. This fuels his persistent intervention against the ghostly Kirara, despite her non-threatening nature and emotional ties to the others.

Oshihara primarily creates narrative conflict by directly opposing the ghost Kirara's attempts to influence past events. His exorcism efforts hinder her mission to guide her younger self and Konpei toward their destined relationship, making him an obstacle born from misunderstanding, not malice.

His involvement underscores themes of misinterpretation; his supernatural expertise fails to distinguish between benevolent and malevolent spirits. His story ends without confirmation of whether he continues his exorcism pursuit or ever recognizes Kirara's true nature.