Manami Amamiya, nicknamed "Manabi," arrives at Seioh Private High School in 2035 as a transfer student during an era of widespread school closures and diminished student spirits caused by declining birth rates. On her very first day, driven by infectious enthusiasm, she spontaneously sings the school song and volunteers for student council president, securing election despite her complete lack of prior experience. Her personal motto, "Massugu Go!" ("Go straight ahead!"), fuels her proactive and optimistic nature, which she channels to inspire others.

Her initial presidency struggles with organizing meetings and delegating tasks, relying heavily on close friends Mika Inamori, Mutsuki Uehara, Mei Etoh, and Momoha Odori for council support. Manami's first major project transforms the student council room into an open communal space welcoming all students, fostering club collaboration and gradually reigniting campus engagement. Her vibrant energy becomes the catalyst for shifting Seioh's apathetic atmosphere. She spearheads organizing the school festival, viewing it as essential for restoring school spirit and demonstrating collective experience value, inspiring peers through rousing speeches and sheer determination. A summer training camp, including pool cleaning that dissolves into a playful water fight, underscores her belief in unstructured bonding alongside academics.

Manami develops by acknowledging her leadership limitations, learning to delegate responsibilities: Mei handles technical tasks like presentations and budgets, while Momoha documents events. Her focus intensifies on ensuring the festival's success as a unifying event. Her influence helps Mei overcome past disillusionment with school activities and encourages Mika to embrace greater responsibility. Later, she and the council confront broader challenges, opposing festival cancellation and advocating for preserving school traditions. A distant finale depicts her graduation, symbolizing her lasting impact on the school's legacy. She leaves behind a revitalized Seioh High where students rediscover the joy of community and shared experiences.

Across all media, including the main series and OVA, she embodies perseverance and youthful idealism's transformative power against institutional decline. She lives under the guardianship of her older brother, Takefumi Amamiya. Her interactions with peers and teachers, like the supportive yet occasionally unreliable Shimojima-sensei, consistently position her as a unifying force within the school.