Junya Nakajo is a minor but memorable character in the story, appearing as a student associated with the Classics Club's film project. He serves as the assistant director for the independent movie being produced by Fuyumi Irisu, a senior student who seeks help from the club to complete an unfinished script. His primary role is as one of three "amateur detectives" brought in to propose solutions for the mystery at the heart of the film's narrative.

In terms of appearance, Nakajo has a distinctive and somewhat mature look. He possesses tanned skin, short brown hair, and brown eyes, with a small amount of stubble on his chin. His build is large, which makes him appear older than his actual age as a high school student. This physical impression aligns with his somewhat gruff and no-nonsense demeanor.

Nakajo's personality is a mix of enthusiasm and irritability. While he may seem unfriendly at first glance, he proves to be a decent individual, though easily frustrated. He is genuinely excited about the film project and expresses that filming was enjoyable, but he also becomes upset when discussing its incomplete state. A defining trait is his straightforward and drama-focused approach to storytelling. He openly values theatrical thrill and viewer engagement over the intricate logical consistency that defines classical mystery fiction. This is most clearly shown when he dismisses complicated trickery, stating that the emotional impact of a culprit's confession is what truly matters, and when he proposes a simple, action-oriented title for the film, "The Furuoka Deserted Village Murder Case".

Nakajo's primary motivation is simply to complete the movie. He is impatient to move past the creative hurdle of the unsolved mystery so that production can continue. His contribution to solving the mystery is notable for its lack of sophistication. His theory for the locked-room murder is that the culprit simply escaped through a window, and he fails to understand the importance of physical evidence like footprints. When the lack of clues is pointed out, he suggests the author might have forgotten to include them. In a moment of improvisation, he latches onto the idea of "summer grass" as an explanation for the inconsistency, hastily concluding that trimming the overgrown grass would solve the plot hole before rushing off to present his theory. This action demonstrates his preference for a quick, pragmatic solution over a well-reasoned one.

His key interactions are with the main members of the Classics Club: Houtarou Oreki, Satoshi Fukube, and Mayaka Ibara. He apologizes to them for getting involved in the project's troubles thanks them for their help, and openly debates his theories with them. However, he quickly becomes irritated when his ideas are questioned, particularly by Houtarou's sharp observations or Mayaka's criticism that his window escape theory is too "lame" for a mystery. Despite his confidence, his reasoning is ultimately dismissed by the club as inconsistent.

Nakajo does not undergo significant personal development, as his appearance is confined to a single arc. His role is primarily functional: to provide a contrasting perspective on mystery-solving. While other characters struggle with logic and character motivation within the script, Nakajo represents a practical, entertainment-first viewpoint. He is neither particularly intelligent nor insightful when it comes to puzzles, but his energetic and somewhat blundering attempts to solve the case add a layer of realism to the group dynamic, showing that not everyone involved in a creative project has a mind for intricate details. His notable ability is his practical knowledge of film production, specifically in props and staging, as he admires the work of the props master and the performance of an actor's death scene.