Sadaharu Inui 乾貞治

A 3rd-year student and regular member of the Seigaku tennis team. He is an intelligent player and is best known for collecting player data, which he incorporates into a unique playing style known as "Data Tennis". Before Echizen Ryoma's arrival at Seigaku, Inui was recognized as the third-strongest player on the team, next to Fuji Shūsuke and Tezuka Kunimitsu. Inui has an intense rivalry with Tezuka, having played at least 30 matches against him and losing every single one of them. In response to Tezuka's superiority, Inui began developing his style of Data Tennis specifically to defeat Tezuka.

Four years prior to the start of the series, Inui and Yanagi Renji were unstoppable Elementary School Doubles Pairing. Despite Inui wanting to continue their close partnership, their doubles pairing was ended when Yanagi suddenly moved to the Kanagawa Prefecture and joined [[|Rikkai Dagaku Fuzoku |Rikkaidai]].

Inui usually plays doubles, most notably with Kaidou Kaoru. His doubling with Kaidou helped Seigaku overcome their problems with their lack of strong doubles teams, with the exception of the Golden Pair.