Masamune Kikuiri is the main protagonist of the anime film Maboroshi. He is a fourteen-year-old middle school student living in the rural town of Mifuse, which becomes completely isolated after a catastrophic explosion at the local steel mill. The disaster traps the town and its residents in a frozen state where time has stopped, seasons no longer change, and no one can age or die. Masamune lives with his father Akihune, a steel mill worker, his mother Misato, and his uncle Tokimune, and he spends his days in a state of deep boredom and emotional stagnation, having grown accustomed to the monotonous, unchanging existence imposed on everyone. The townspeople are required to submit periodic identification forms to prove they have not changed in any way, and they are forbidden from altering their identities or expressing strong emotions, as doing so can cause cracks to appear in the sky and lead to a person vanishing.

Masamune is characterized by his suppressed emotions, quiet frustration, and a growing but initially dormant desire for something real to break the endless repetition. He is resigned to his empty life until his enigmatic classmate Mutsumi Sagami leads him to the fifth furnace of the steel mill, where he discovers a feral girl who cannot speak and behaves like a wild animal. Masamune names the girl Itsumi and, together with Mutsumi, begins secretly caring for her. This encounter awakens feelings he had long buried and becomes the catalyst for his gradual transformation. His motivation shifts from passive acceptance to a determined desire to understand the truth behind the town's condition and to free Itsumi, who he learns is actually a real girl named Saki Kikuiri transported from the outside world. Masamune's growing attachment to Itsumi and his deepening connection with Mutsumi lead him to rebel against the rigid rules enforced by figures like Mutsumi's father Mamoru Sagami, who insists that maintaining the status quo is necessary for survival.

Masamune's key relationships include his complex bond with Mutsumi, who challenges his passivity and shares a tense, romantic undercurrent with him, and his protective, almost paternal connection with Itsumi, who depends on him and Mutsumi as surrogate guardians. He also maintains friendships with classmates Daisuke Sasakura and Atsushi Nitta, who join him in disrupting the town's order. His relationship with his father Akihune is marked by quiet tension, as Akihune initially enforces the town's rules but eventually reveals through a journal that he had discovered the truth about Itsumi and the constructed reality of Mifuse, inspiring Masamune to take action. His uncle Tokimune provides a more skeptical perspective on the town's leadership and later aids the protagonists.

Throughout the story, Masamune develops from a listless, resigned teenager into someone willing to embrace the pain of change and reality over the safety of an illusory, frozen existence. He learns that even in a world where time has stopped, people can still grow and change internally. His most notable actions include enlisting his classmates to disrupt the sacred machine's ritual, rescuing Itsumi from being offered to the mill, and placing her on a freight train bound for the real world. Masamune does not possess supernatural abilities; his significance lies in his emotional awakening and his choice to challenge the town's stagnation, ultimately accepting the possibility of loss and an uncertain future in order to give Itsumi, and by extension himself, a chance at a real life.