Naoya Yamashita is a supporting character in the film My Oni Girl. He is a classmate of the protagonist, Hiiragi Yatsuse, and attends the same high school in Yamagata Prefecture. Naoya is portrayed as a casually opportunistic individual who habitually takes advantage of Hiiragi's inability to refuse requests. He often asks Hiiragi to help with homework, carry items, or perform other small favors, assuming agreement without considering the burden on Hiiragi. His demeanor is friendly and laid-back, which masks a self-centered and thoughtless nature. Naoya views himself as reliable but effectively delegates tasks to keep his own time free, disliking being told no and preferring easygoing friends who will assist him without complaint. His primary motivation appears to be convenience and maintaining a carefree social life without taking on tedious responsibilities.

In the story, Naoya functions as a narrative foil that establishes Hiiragi's early character flaw: his desperate need to please others and avoid conflict. Through their interactions, the film highlights the social pressure Hiiragi faces at school and the toxic dynamic of one-sided friendships. Naoya's casual dismissal of Hiiragi's feelings underscores the unhealthy environment that Hiiragi must eventually break away from. While Naoya is not a villain, his behavior represents the casual exploitation that people-pleasers like Hiiragi endure.

Naoya's key relationship is with Hiiragi Yatsuse, whom he sees as a reliably helpful peer. He also interacts with other classmates such as Mio, but these relationships are similarly superficial and centered on his own convenience. Naoya does not undergo significant character development over the course of the film; his role remains static as a representation of the social pressures that Hiiragi learns to overcome. He has no notable supernatural abilities and exists entirely within the human world, grounded in the mundane school setting that contrasts with the mystical journey Hiiragi undertakes.