Chihiro Shindō is the younger twin sister of Kei Shindō, designated as such by Japanese convention despite being nearly the same age. A childhood car accident caused significant brain damage and the loss of her left eye, resulting in severe anterograde amnesia. This irreversible hippocampal injury confines her lived experience to a recurring thirteen-hour cycle, permanently preventing new long-term memory formation while leaving pre-accident recall intact. She meticulously records daily events in a diary, her essential lifeline to the past. Paradoxically, her short-term recall within each thirteen-hour window remains exceptionally sharp. Her memory impairment is metaphorically depicted as an illuminated circle of glass. Due to an adverse reaction causing panic, she wears a replaceable paper eyepatch instead of a prosthetic eye.

Residing in Otowa, Australia under the guardianship of Yuu Himura—whom she knew pre-accident as "big brother" despite no biological relation—she lives in recuperation and does not attend school. Once outgoing and active like her twin, Chihiro is now reserved, quiet, and introverted. Her personality encompasses shyness, kindness, stoicism, stubbornness, and low self-esteem, often making her seem taciturn or flustered socially. She possesses a bookish disposition, harbors a strong passion for reading novels, and aspires to become a writer, though she struggles with cooking. Habitual mannerisms include glancing upward during conversations and tilting her head.

Her development centers on meeting Renji Asou daily at an abandoned train station. Learning of her dream to write a novel, Renji collaborates with her. Their writing process becomes an allegory for her life and fragmented perception. Chihiro documents her growing affection for Renji in her diary, but the distinction between recorded feelings and actual relationship progression sometimes leads her to overinterpret their closeness, occasionally exacerbated by Renji's initial shyness, which she misinterprets as rejection. A pivotal realization occurs when she understands she can preserve her feelings for Renji by consciously thinking about him within each thirteen-hour window, allowing those emotions to persist indefinitely. This emotional breakthrough metaphorically shatters her mental constraints, leading to reconciliation and the establishment of a committed romantic relationship with Renji.

Across the broader narrative, particularly in later installments, Chihiro undergoes noticeable physical maturation into a more womanly figure, symbolizing her emotional growth and resilience in confronting her condition. Her story concludes with a sense of emotional liberation achieved through her relationship with Renji and fulfillment in collaborative writing, framing her journey as one of overcoming psychological barriers despite the permanence of her neurological condition.