Kengo Haryu is a student at Eimei High School and a key member of its badminton team. Initially a second-year student in class 2-B, he later advances to the third year. He has dark brown hair and is in the same class as Chinatsu Kano, sitting next to her and serving as a class representative alongside her. Kengo is recognized as one of the most talented badminton players at the school, and his skill and potential mark him as a strong contender for the national championships, which he qualifies for as a singles player during his second year.

His personality is defined by a strict and demanding exterior that masks a deeply caring and supportive nature. As an upperclassman, he is particularly hard on Taiki Inomata, a younger teammate. This severity is not born from malice but from high expectations; he recognizes Taiki's potential and pushes him relentlessly to help him grow. This tough-love approach extends to his role as a mentor, as he actively guides Taiki, corrects his techniques, and even takes him to practice with college-level players to improve his skills. Beyond the court, he shows a caring side and is known to be protective of his peers.

Kengo's primary motivation is his own success in badminton, driven by a fierce and determined competitive spirit. His goal to reach the national tournament is a constant force in his life. However, his motivations are not purely selfish. He is invested in the success of the badminton club and takes his responsibility as a senior seriously, wanting to see his juniors, especially Taiki, succeed. He is shown to believe that Taiki will eventually qualify for the Interhigh, a major national tournament, even when others have different expectations.

In the story, Kengo serves as a crucial character who bridges multiple dynamics. He is the senior and doubles partner to Taiki Inomata, acting as both a rival and a mentor. His relationship with Taiki evolves significantly from one of intimidating seniority to a deep mutual respect. He is also a classmate of Chinatsu Kano, the main heroine, and their close friendship initially creates a point of romantic tension for Taiki, though this is quickly resolved when Kengo reveals he has a girlfriend. This relationship is a central part of his personal life; he is dating Karen Moriya, whom he has known since the fifth grade. They met at a swimming class, and their bond is built on a mutual admiration for each other's dedication to their respective goals. Their relationship operates on an understanding that they will not let their romance interfere with their ambitions, a condition they set for themselves when they began dating. Kengo is also a former member of the KTS Junior team, where he was partners with Shoichiro Kishi.

Throughout the series, Kengo undergoes notable development, particularly in his role as a mentor. While he is initially a formidable figure for Taiki to look up to and surpass, his own journey includes facing defeat. He has a significant rivalry with a player named Hyodo, the national singles champion, whom he faces in intense matches. Despite losing a close and hard-fought match against Hyodo during the nationals, Kengo demonstrates resilience and a desire for revenge, showing that his spirit is not broken by defeat. Eventually, he passes the torch to the next generation, as Taiki manages to defeat him in a qualifying match during Kengo's third year.

As a player, Kengo possesses notable badminton abilities. His technical skills and strategic play are of a high enough caliber to compete at the national level. He is known for playing defensively in crucial matches, a trait that is both a strength and a tactical choice that his opponents analyze. He becomes the vice-captain of the badminton club after the change in team generations, solidifying his leadership role within the team.