Kotetsu Tatejima is Kohane Hatoya's classmate at Kaminoki High School. Naturally timid and reserved, she grapples with significant confidence issues. This lack of self-assurance stems partly from embarrassment over her name, which she perceives as overly masculine and unrefined due to its association with a famous historical blacksmith. She is also sensitive about her physical appearance and weight.

Despite her reserved nature, she occasionally displays unexpected bluntness in conversations. Performance situations, like piano recitals, severely challenge her confidence, causing her to become easily flustered and lose composure. This fear of embarrassment initially made her reluctant to join cheer activities. However, she demonstrates genuine care for her friends, deeply considering how to support Kon Akitsune during Kon's confession rather than offering superficial advice.

Her connection to the Cheer Association begins indirectly while seeking a quiet place to practice piano in the supposedly haunted AV room, where she encounters the core members. Observing their dedication—particularly their efforts creating homemade uniforms and pom-poms due to budget constraints—sparks a quiet interest. Witnessing their energetic and successful cheering performance at a crucial basketball game, rallying the entire school, becomes pivotal. This demonstration of collective spirit helps her realize cheering transcends individual performance and is about shared support. Consequently, she tentatively expresses her desire to join, motivated by the supportive atmosphere.

After joining, she undergoes gradual development. The other members hold a welcome party to celebrate her inclusion. She begins training under Hizume Arima's guidance, starting with fundamental warm-ups and mastering basic cheerleading arm motions while focusing on maintaining a smile. Adapting to the cheer uniform takes time, reflecting her ongoing journey towards comfort within the group. Her comedic role solidifies as the frequent "tsukkomi" (straight man), offering deadpan reactions or corrections to others' antics—a tendency so ingrained it sometimes activates even when the source of the absurdity is elsewhere.

Participation in cheerleading catalyzes her personal growth. The supportive environment and collaborative nature steadily help build her confidence. Facing challenges, like overcoming nervousness during a festival performance where the group executes a complex Double Base Thigh Stand routine with Kohane at the top, contributes to her increasing resilience and ability to manage anxieties in front of crowds.