Ken Wakashimazu is introduced as the goalkeeper for the Meiwa FC elementary school team, where he first appears during the national tournament semifinals. He is the youngest son of a karate dojo family, specifically the Wakadoryu style, and has trained in karate from a young age. Because of this background, he is often called the karate goalkeeper. Before becoming a goalkeeper, he originally showed interest in playing as a forward. However, a car accident in which he saved a dog resulted in a broken shoulder and leg, delaying his debut until the semifinal match against Furano FC. In that game, he immediately made an impact by saving a penalty kick from Hikaru Matsuyama.

Wakashimazu possesses a calm and cool-headed personality but can also display a fierce and intense demeanor on the field. He is highly disciplined, strict with his training, and proud of his abilities. This pride is a defining trait, as he becomes deeply offended if his skills as a player are undervalued. He is also a man of few words, reserved in normal situations, and deeply faithful to his close friends and teammates.

His primary motivation stems from a deep-seated rivalry with Genzo Wakabayashi, whom he considers the ultimate benchmark for a goalkeeper. This rivalry pushes him to constantly improve. In his early years, a major personal goal is to win the national tournament and defeat Wakabayashi. This becomes even more critical when his father, who wants him to inherit the family dojo, agrees to let him continue playing football only if he can win the championship. His other strong motivation is supporting his best friend, the ace striker Kojiro Hyuga, and helping their team achieve victory. After following Hyuga to Toho Academy, Wakashimazu serves as the team's last line of defense and, on occasion, its captain.

Wakashimazu plays a central role as one of Japan's top goalkeepers. He is portrayed as the main domestic rival to Genzo Wakabayashi, who plays in Germany. Throughout the junior youth arc, he is the starting goalkeeper for the Japanese national team. In the story, his character undergoes significant development. Initially somewhat conceited regarding his abilities, he learns to acknowledge the strengths of others. His pride leads to a major conflict with the national team coach over the starting goalkeeper position, causing him to temporarily leave the team. He eventually overcomes this, returns, and accepts a role that supports the team's success. He also demonstrates a willingness to adapt, later transforming into an attacking player or forward in later storylines, showing his versatility and commitment to the team above his personal position.

His key relationships include his strong friendship and partnership with Kojiro Hyuga, having played together since childhood. He is also very close to Takeshi Sawada, a teammate who follows him and Hyuga to Toho. His fierce rivalry with Genzo Wakabayashi is another cornerstone of his character arc, representing the competition for the number one spot in Japan. He also shares a significant bond with his coach, Kozo Kira, who originally helped bring him into football.

Wakashimazu's notable abilities are all derived from his karate training. His signature move is the Triangle Kick (Sankaku Geri), which uses the goalpost to launch himself in mid-air to make saves he would otherwise miss. He also uses the Hand Sword Defense (Shuto Defense) to strike and deflect powerful shots. Another technique is the Seiken Defense, a straight punch used to fend off the ball. He has excellent leaping ability and quick reflexes, and his powerful overhand throw allows him to start counterattacks with great speed and distance. As a goalkeeper, he is known for being an attacking type with a large defensive range, contrasting with Wakabayashi's more positionally perfect, defensive style.