Chihiro Shindō is the younger twin sister of Kei Shindō. When she was twelve years old, she was involved in a severe car accident that resulted in the loss of her left eye and inflicted significant brain damage. This injury caused a form of anterograde amnesia, which prevents her from forming new long-term memories. Her memory resets every thirteen hours, leaving her unable to recall anything that happened after the accident beyond this short window, though her memories from before the accident remain intact. To manage her condition, she wears a simple paper eyepatch and lives under the care of Yuu Himura, whom she has known since childhood and calls "big brother," in the town of Otowa.

Chihiro has a quiet, reserved, and introverted personality, a stark contrast to her more outgoing twin sister. She is characterized by her shyness, kindness, and a deep-seated low self-esteem, which often leads her to feel like a burden to others. Despite her fragile demeanor, she possesses a strong, stubborn dedication to literature. An avid bookworm, she harbors a passionate dream of becoming a writer, a goal that feels almost impossible given her memory condition. Her life is anchored by a diary in which she meticulously records the events of each day. Every morning, she rereads the diary to understand what has happened in her life, relying on the written words as a substitute for genuine memory.

Her life changes dramatically when she meets Renji Asou at an abandoned train station where she often reads alone. Renji becomes the central figure in her story, as he learns of her condition and her dream to write a novel and decides to help her. Together, they collaborate on writing a story that serves as an allegory for her own life, trapped in a static world of memory. As their relationship deepens, Chihiro records her growing affection for Renji in her diary. However, her reliance on the diary creates a unique struggle; she sometimes confuses the written records of her feelings with her direct experiences, leading to misunderstandings about the state of their relationship.

Throughout the narrative, Chihiro struggles with feelings of worthlessness and isolation brought on by her disability. This culminates in a crisis where she threatens to harm herself and tears up her diary pages, an act meant to erase the current version of herself. The key to her development lies in a profound realization about the nature of her memory: while her explicit memories of events disappear, her emotions can be sustained if she consciously reinforces them. By choosing to think of Renji constantly within her thirteen-hour window, she is able to preserve her feelings for him indefinitely, effectively breaking the metaphorical stained-glass prison of her temporal isolation. This breakthrough allows her to reconcile with Renji and commit to their relationship despite the permanence of her amnesia.