Kazumasa Nakaoka is a twenty-one-year-old man who works as a part-time employee at a motorcycle shop. His appearance is notably marked by his long hair, which contrasts with his history as a disciplined and celebrated athlete. In his past, he was a star forward for the Teiho High School soccer team, known by nicknames such as the "miracle lone striker" and "future ace." He was scouted to join the professional J.League team Tokyo Spirits, but his career was derailed by a motorcycle accident that left him with a leg injury.

Before his accident, Nakaoka met a young, frail boy named Tomofumi Motoura, the son of a soccer coach. Despite his weak constitution, Tomofumi was passionate about soccer and begged Nakaoka to teach him. Nakaoka was deeply moved by the boy's determination, and the two formed a close bond as mentor and student. Following his injury, Nakaoka lost contact with Tomofumi for a few years. When they reunited, Tomofumi was in the fourth grade and had joined his school's soccer team, though his fragile health kept him from playing in matches. When Tomofumi was finally given a chance to play and scored a goal, Nakaoka promised to celebrate with him. However, on the day of their planned celebration, Nakaoka learned from Tomofumi's mother that the boy had been involved in a fatal accident on his way home from practice.

This tragic event became the catalyst for Nakaoka's actions. From Tomofumi's father, he learned that the ambulance carrying the boy to the hospital had been delayed by a mob of people who had blocked its path. Nakaoka's grief and resentment turned to focused bitterness upon discovering that a drunken Kogoro Mouri was the leader of that mob. Convinced that this delay led to Tomofumi's death, he decided to dedicate the next several years to meticulously plotting his revenge against Kogoro.

Nakaoka possesses a complex personality driven by a rigid personal code. He is characterized as stubborn, sensitive, and prone to melancholy. His fundamental belief, rooted in his own pride as an athlete, is that a soccer player who cannot play an entire ninety-minute match is not a true player. This belief underscores his refusal to accept Tokyo Spirits' offer to join the team even after his injury, as they had proposed using him for limited fifteen-minute appearances. His pride would not allow him to accept such a role. This same inflexible logic becomes a key part of his vengeful plan, which involves a series of riddles and bombs set across multiple stadiums. His desire is to confront a final "striker" who can end his revenge, a role he initially intends for Kogoro. His grief over Tomofumi, a boy who could only play soccer in short bursts, ultimately corrupts his own philosophy.

In the story, Nakaoka is the main antagonist, orchestrating a complex scheme of bombings and coded messages specifically targeting Kogoro Mouri. His role is that of a tragic villain, whose deep friendship with a child and inability to process his death drove him to extremism. Key relationships defining his character include his past friendship with Tomofumi Motoura, for whom he felt a profound responsibility and affection. His relationship with Kogoro is one of pure, misguided revenge, holding him solely accountable for the boy's death. In a confrontation with Conan Edogawa, who deduces his identity and plan, Nakaoka is forced to confront the flaw in his ideology. Conan argues that if a player cannot play for ninety minutes, they are not a real player, then by that logic, Tomofumi was not a real soccer player either, and that Nakaoka's own time on the field, no matter how short, should be used to its fullest.

Through this encounter, Nakaoka experiences a development in his perspective. Upon witnessing Conan's skill and determination to save everyone, he experiences a shift, recognizing Conan as the "eleventh striker" he was searching for to bring an end to his destructive path. His notable abilities are centered on soccer. He was a highly talented forward with exceptional skill and a deep tactical understanding of the game. He also demonstrates a capable technical mind, creating a complex network of bombs and a series of sophisticated riddles that challenge both the police and the protagonists. Despite his physical injury ending his professional career, his athletic knowledge and physical prowess remain a significant part of his character.