Sadaharu Inui stands 184 cm tall as a third-year Seishun Academy student with a lean build. Thick-framed glasses obscure his eyes, defining his distinctive look. His personality revolves around relentless data collection; he constantly fills notebooks with observations on tennis strategies and intimate details of teammates and rivals, even spying outside school. He hones "Data Tennis," predicting opponents through statistical analysis—a method born from his desire to defeat Captain Kunimitsu Tezuka after losing every one of their 30+ matches. Before Ryoma Echizen's arrival, Inui ranked as Seigaku’s third-strongest player behind Tezuka and Fuji. His elementary school doubles dominance with Yanagi Renji ended when Yanagi transferred to Rikkaidai. Temporarily removed from the regular lineup following losses to Echizen and Kaido Kaoru, Inui served as team manager. He designed punishing training regimens and created "Inui Juice," a harsh vegetable blend consumed willingly only by himself and Fuji. He later reclaimed his regular spot by defeating Momoshiro Takeshi using updated data. Inui employs a Serve and Volley style, specializing in high-speed serves like "Sonic Serve" and "Waterfall." Primarily a singles player, his doubles partnership with Kaido bolstered Seigaku’s roster beyond the Golden Pair. Data Tennis falters against unpredictable players like Echizen or those concealing abilities like Tezuka and Fuji. At the U-17 Camp, Inui forfeited a tie-breaker due to stomach issues from a miscalculated juice ingredient. After mountain training, he helped defeat Court 2. During the Camp Revolution, he urged Yanagi to abandon data dependence against Mitsuya Akuto, substituting when Yanagi collapsed. His family includes public officer parents. Proficient in physics but not home economics, he excels at strategy games like Othello and Go. He spends his allowance on notebooks and reference books, frequents the school’s third archive room, and prefers calm, older women—his first love lasted 4.2 seconds. In the Nationals finals against Rikkaidai, Inui partnered with Kaido against Yanagi and Kirihara Akaya, losing the match. His equipment features a Prince Graphite Challenge racquet and Puma Sequence PT0636 0067 shoes. His bag carries data tools: a laptop, calculator, voice recorder, match tapes, and his secret notebook.