Aki Hayama is the female protagonist of the story and a first-year junior high school student living in Chofu, Tokyo. At the start of the narrative, she takes the surprising step of confessing to her fellow freshman, Makoto Natsume, whom she has apparently just met. Her declaration is rooted in a mysterious family legend; Aki explains that her grandmother once told her she would meet a boy named Makoto Natsume in the future. This belief is connected to a painting of a fireworks display that Aki possesses, which she believes holds the key to finding the Makoto Natsume from her grandmother's story.

Aki is characterized as a popular and talented student, also serving as her class president. She is described as being beautiful and skilled, presenting a confident and capable exterior to her peers. Her initial confession to the shy and socially awkward Makoto, a boy she has never formally met, reveals a determined and possibly idealistic nature, as she is driven by a promise that transcends her own memory. Her motivations are deeply personal and tied to her family history; she is on a quest to fulfill or understand the enigmatic promise made to her grandmother, which links her to a boy named Makoto Natsume.

In the story, Aki serves as the catalyst for the central mystery and journey. Convinced of the connection between the fireworks painting and the unknown Makoto, she enlists the help of the boy who shares that name. Together, they investigate the secret of the painting, which is said to contain an important promise made long ago. Her unwavering belief in her grandmother's words directly influences Makoto, prompting him to assist her in her search. This search leads them to travel from Tokyo to the city of Nagaoka in Niigata Prefecture to attend its famous fireworks festival, which they suspect is the location depicted in the painting. Her primary relationship is with Makoto Natsume, as the pair are bound together by the mystery of the promise, and her past is directly linked to his name through her grandmother.