Junpei Kasahara is a third-year student at Seiseki High School and a member of its soccer club. His decision to leave the team is a central part of his story, initially expressing a feeling of limitation as his reason for wanting to quit. The true reason, however, is the declining health of his grandmother, who takes care of him. He decides to make the Inter-High tournament his final competition before retiring from the club to transfer to a school closer to her home and care for her.

As a player, Kasahara is known for his meticulous and accurate shooting, as well as a calm mental state that makes him reliable, particularly in the final moments of a match. He is described as very reliable, mentally strong, and one of the hardest workers on the team, dedicating significant time to weight training and film study. This approach is characterized as a safety-first mentality, which, while dependable, means he lacks the boldness to be a decisive game-changer.

Despite his reliability, Kasahara was not named in the seventeen-strong squad for the Inter-High tournament, with his spot going to the underclassman Tsukushi Tsukamoto. He fully supported Tsukushi from the sidelines during the qualifiers. In the final of the Inter-High qualifiers, he had full confidence that he himself would have scored one hundred percent on a particular last shot, but he acknowledged that it was Tsukushi who managed to get into position and take that crucial shot.

His early days in the club were not easy; he was part of the same struggling group as Hisahito Mizuki. Together, they endured rigorous training, practicing in all weather conditions. Kasahara holds great respect for Mizuki, who went on to become a representative player for Tokyo, unlike himself. After his own retirement, he continues to follow Seiseki's matches, often watching them with Kiichi Ooshiba's older sister, and is amazed by the growth of players like Tsukushi and Ooshiba. He is also called Cassano-senpai by Jin Kazama.