Yoshino Kawaguchi attends Shin Koshigaya High School as the twin sister of Ibuki Kawaguchi, recognizable by her medium-length blonde hair and blue eyes. She possesses significant baseball knowledge, immediately identifying Yomi Takeda's pitching background and repertoire by examining her hand during their first classroom meeting. Yoshino explains the school baseball club is inactive, limiting playing chances, despite Yomi stating she quit baseball.

After school, Yoshino accompanies Yomi and Ibuki, witnessing Yomi reunite with childhood friend Tamaki Yamazaki. Recognizing Tamaki as a famous catcher from Minami Girls, Yoshino requests her autograph. She then suggests a casual game of catch that evolves into pitching practice between Yomi and Tamaki. Observing their strong compatibility and shared enthusiasm for the game, Yoshino persuades both to join the baseball club and volunteers as manager. Her initiative directly revives the club after its hiatus caused by an unspecified scandal.

Her surname Kawaguchi means "mouth of the river" (川口). Her given name Yoshino combines "fragrant, virtuous, beautiful" (芳) with "from" (乃). Yoshino shares a close bond with her sister Ibuki; they frequently appear together at school and collaborate to recruit members for the baseball team. As manager, Yoshino actively supports the club's goal of reaching the national baseball competition.