Jūjirō Oni is a high school student competing in Japan's U-17 training camp. He secures a spot as an All-Rounder on the Japanese U-17 World Cup team roster. Initially assigned to Court 5 during the camp's court placements, this ranking belies his true capability. Members of the elite 1st Court openly recognize his considerable power and skill. Oni shares this trait of being under-ranked with Kanata Irie, who occupies Court 3. Following a court shuffle triggered by the 5th Court defeating the 3rd Court—a match where Irie feigned an arm injury and could not play—Oni ascends to Court 3. His formidable presence and role within the camp structure earn him the nickname "The Gatekeeper from Hell." Within the team, he stands as one of only two high school participants not in their third year, alongside Kazuya Tokugawa. Oni is selected for Japan's representative team at the U-17 World Cup. During the semi-final clash against Germany, he takes the court in the Singles 3 position against Quark Puppe. The match extends to three sets, concluding with Oni's defeat at 6-7, 6-3, 3-6. His participation in this critical semi-final singles match underscores his standing as a key player trusted in high-stakes situations.