Sekine is a schoolteacher who appears as the teacher of Izumi Toyama and several of her classmates, including Chiharu, Miyuki, Hiromi, Saori, Chiaki, and Ayano. His personal background is not explored, and he appears only in his professional capacity within the school setting. During a school trip, he accompanies the students and takes photographs with them, an ordinary activity that later takes on significant meaning.

His main role in the narrative is tied to those photographs. After the trip, the supernatural curse that has touched the students begins to alter the developed images, and the faces of the students who have been claimed by the curse appear eerily blackened. The photographs show the same corruption spreading to Izumi once she too becomes marked. In this way, Sekine unknowingly provides one of the story's clearest visual signs of how the curse leaves its mark on its victims.

Sekine's personality and motivations are not developed in the story, and he does not undergo any character growth. He functions as a minor background figure whose significance comes entirely from the photos taken during the trip, which act as evidence of the curse's influence. His relationships are limited to his professional connection with the students, and he displays no special abilities. His contribution to the story is therefore indirect but notable, as the corrupted photographs reveal the fate of the students he supervised.