Kouji Sekine is a young man living in the Kyoto region who works as a cameraman and is also a professional karuta player. In the competitive karuta world, he was originally a member of the Nagoro-Kai, an elite club limited to only twenty members, where he trained under the respected mentor Shikao Nagoro. After Nagoro disappeared, Sekine moved to the much larger Satsuki-Kai, the organization that runs the Satsuki Cup tournament. Despite his skills, he repeatedly reached the finals of the Satsuki Cup but lost each time to the reigning champion, Toshiya Yajima, leaving him with a strong sense of rivalry and lingering resentment. Within the Satsuki-Kai, he also served as the organizer of the group's annual lottery.

His personality is a mixture of sharp intelligence and a certain cocky arrogance. He thinks quickly under pressure and can devise clever ways to deflect suspicion, such as preparing a fake alibi or carefully withholding incriminating details. However, when he becomes overconfident, he tends to slip up and betray information he should not know, which then makes him feel cornered. In stressful moments, he has a habit of pacing in circles. He generally carries a snarky, dismissive attitude, but often tries to mask it with an innocent expression. He speaks in the Kansai dialect, reflecting his regional background. Physically, he is tall and slim, with a distinctive large, pointed nose and short, somewhat sassy blond hair. His attire consists of suits: on one occasion a relaxed grey jacket over a green shirt and blue trousers without a tie, and on another a more formal tan suit with a grey shirt and a black necktie.

His involvement in the film's central case begins when he discovers Yajima's body before the murder is publicly known. Panicking, he hides his presence at the scene and switches off his mobile phone, later arriving at the crime scene and pretending to have been drunk and unreachable. This clumsy performance immediately draws the attention of Heiji Hattori and Conan Edogawa. During a tense confrontation, Sekine accidentally reveals that Yajima was beaten to death, a detail that only someone with inside knowledge of the murder weapon would know, cementing the detectives' suspicions. Although he manages to evade immediate consequences, the real culprit soon targets him: a bomb is planted under his car, and he is gravely injured in the explosion, though he ultimately survives and awakens in hospital care.

Sekine's relationships are shaped by the competitive karuta circuit and the investigation. Toshiya Yajima was his foremost rival; Sekine's repeated failures against him fueled a suppressed bitterness, and Yajima's death opened a path for him to finally claim the Satsuki Cup title. With Kensuke Achiwa, the Satsuki-Kai president, he maintained a seemingly cordial bond, but Achiwa actually manipulated Sekine as part of his larger scheme, using him as a convenient pawn. He holds deep respect and loyalty for his former teacher Shikao Nagoro, whose legacy continued to influence him. Although he and the detectives clash, by the end of the case he is no longer treated as a culprit but rather as a near-victim of the true killer. Kouji Sekine is a character defined by his competitive drive, his quick but flawed mind, and his entanglement in a murder he did not commit, serving as a red herring whose survival ultimately underscores the danger of the deadly plot.