Akane Aoi is the elder sister in the Aioi family and a central figure in the story, whose life has been shaped by sacrifice and a deep sense of responsibility. Her background is defined by a tragic event in her late teens when both parents died in a car accident. This sudden loss forced her to abandon her personal dreams, specifically her ambition to move to Tokyo with her boyfriend, Shinnosuke Kanamuro, to pursue a future in music. Instead, she chose to remain in her rural hometown of Chichibu to become the surrogate parent and primary guardian for her younger sister, Aoi.

Thirteen years after this pivotal decision, Akane has built a stable and respectable life for herself and her sister. She works a practical, unglamorous job at the municipal office, where she is an enthusiastic and valued member of the community. Her role in the story is that of the responsible, grounded elder sister who holds everything together. She is often involved in local initiatives, such as coordinating a music festival intended to revitalize the town's tourist industry. This event serves as the catalyst for the film's main conflicts, as it brings her estranged ex-boyfriend, Shinnosuke, back to town and leads to the mysterious appearance of a younger version of him from the past.

Personality-wise, Akane presents a facade of bright, unshakeable positivity. She is described as effervescently popular with the townsfolk and maintains a cheerful demeanor, encouraging her younger sister to find beauty in their surroundings, such as the autumn colors on the mountains. However, this outward resilience masks a deep well of unresolved regret and quiet vulnerability. Beneath her composed and pragmatic surface, she harbors the pain of the paths she could not take and the sacrifices she has made. Her motivations are rooted in her role as a protector; her primary driving force for over a decade has been ensuring the stability and well-being of her sister, Aoi, often at the expense of her own happiness and aspirations.

The character's development is closely tied to her key relationships. Her bond with her sister, Aoi, is initially strained, as the younger sister feels both indebted to and resentful of Akane for her sacrifices. Over the course of the story, their relationship transforms as Akane learns to shed her overprotective tendencies, allowing for a new dynamic of mutual respect and understanding. Her relationship with Shinnosuke is also central; it is marked by the tension of their abrupt separation years ago. The return of the adult Shinnosuke, alongside the sudden appearance of his idealistic teenage self, Shinno, forces Akane to confront her long-buried feelings and the "what ifs" of her past. These interactions fracture her carefully maintained composure and guide her toward a fragile reconciliation with her former love, ultimately leading to their marriage and symbolizing her journey toward reclaiming her own agency. Regarding notable abilities, Akane possesses no supernatural or special skills. Her most significant abilities are her quiet resilience, her organizational competence as a civil servant, and her emotional strength in managing family obligations and personal regret.