Koji Imai is a fifth-grade elementary school student in class 5-2, where he holds the class number 3. He is the best friend and primary rival of the protagonist, Ryota Sato. As a character, Koji is bold, quick-witted, and notably proud, always presenting himself as confident and knowledgeable, especially regarding romance and what girls expect from boys. This self-assuredness leads him to frequently challenge Ryota to various duels and competitions in an effort to prove who is better. However, his pride often sets him up for failure, as his attempts to act cool frequently fall flat, leading to regular and spectacular moments of embarrassment. He has a well-documented fear of Yuki Asano's cooking. Within the story, Koji serves as Ryota's foil, with their friendly rivalry and the resulting pranks and arguments driving much of the comedy. His main motivation is to one-up Ryota, yet their bond remains a strong and enduring friendship. He is also known for his surprisingly sharp understanding of how to impress girls, though his schemes rarely go as planned. His abilities include quick thinking in devising challenges, and he harbors a distinct fondness for super balls. True to the comedic and episodic nature of the series, Koji remains a consistently competitive and entertaining presence within the group, showing little personal change.