Shizuka Tsukubae is the younger twin sister of Tomoe Tsukubae. Both sisters were members of the newspaper club before joining the baseball team. Shizuka consistently wears traditional Japanese clothing, contrasting with Tomoe's preference for Western-style sailor uniforms, and they share a dormitory room.

Shizuka possesses a serious, duty-oriented personality focused on responsibility and proper conduct. She initially opposed Tomoe joining the baseball team, deeming it unsuitable. When Tomoe joined anyway, Shizuka followed primarily to stay close, expressing concern about Tomoe managing without her. This decision also stems from Shizuka's tendency toward jealousy when Tomoe directs attention toward others, such as Kyouko Sakurami's pursuit of Tomoe or Tomoe's growing friendship with Koume Suzukawa.

Her protectiveness often manifests as possessiveness regarding Tomoe's interactions. Shizuka remains skeptical of Tomoe's baseball involvement and feels discomfort when her sister forms close bonds outside their twin relationship. Noriko Owari, the newspaper club president, observes this dynamic and notes Tomoe's romantic interest in Koume.

As a baseball player, Shizuka serves as the team's first baseman. She became the sixth team member after Tomoe joined, prioritizing her sisterly connection over personal interest in the sport.