Shōko Makinohara is a central character whose existence spans multiple timelines and age groups due to her connection with Adolescence Syndrome. Born with a terminal heart condition, she endured frequent hospitalizations starting in elementary school, which left her unable to envision a future during a class assignment about life plans.

She first met Sakuta Azusagawa on Shichirigahama Beach during his middle school years, becoming his first romantic interest. There, she shared her philosophy that life's purpose lies in cultivating kindness each day, forging their initial bond. Later, Sakuta and Mai Sakurajima discovered a middle school-aged Shōko sheltering an abandoned cat with her umbrella on a rainy day. Identical to Sakuta's first love yet younger, this Shōko began visiting Sakuta's home to care for the cat, Hayate, though her visits eventually ceased due to her worsening health.

An older version of Shōko, appearing as a high school or university student, manifested during Sakuta's times of emotional distress. She treated the scars on his chest after his sister Kaede's memory loss, revealing they resulted from two versions of his heart existing across timelines—a consequence of her future dependence on his transplanted heart. This older Shōko later requested to live with Sakuta and Mai, explaining her terminal illness fueled her dream of experiencing adulthood. Her presence created a paradox where Sakuta was destined to die in an accident so his heart could save her.

Her Adolescence Syndrome originated from conflicting desires about her future, fracturing her into three distinct versions: an elementary school student halting time in despair over her illness, a middle school student living normally, and a college-aged student whose accelerated time perception projected her into the future. These versions coexisted across divergent timelines and could interact through shared dreams.

In one timeline, Sakuta's girlfriend Mai sacrificed herself to prevent his accident, donating her heart to save Shōko. An older Shōko from this future then helped Sakuta return to the past to prevent Mai's death, revealing she married that timeline's Sakuta. Ultimately, Shōko intervened to create a new reality where Sakuta and Mai survived together. Her heart condition resolved through an unrelated donor, facilitated by Mai's public influence inspiring organ donation. In this revised timeline, Sakuta and Mai retained their memories and encountered a healthy younger Shōko with her family on the same beach, recognizing each other.

Following her recovery, Shōko moved to Okinawa for a fresh start, maintaining contact with Sakuta and sharing updates like a photo of her tanned self. She later noticed timeline anomalies, such as a missing music video by another character. Her story concludes with her continuing to subtly influence events while living independently.