Aya Tokiwa is a character who appears in the high school arc of the story, introduced several years after the events of the middle school narrative. She is a student at the same high school as the protagonist Takao Kasuga, and their initial encounter takes place in a small bookstore where she is seen holding a copy of the book The Flowers of Evil, the same collection of poetry that first sparked Kasuga's interest in literature years prior. This moment becomes the catalyst for their eventual connection.

In terms of personality, Aya is characterized by a sharp contrast between her outward social persona and her inner private world. To her classmates, she appears as a sociable, easy-going, and popular girl, someone who is well-liked and admired by her peers. However, this exterior masks a deeply introspective and literary nature. She is, at her core, a literary girl with a passion for reading, and she secretly harbors a desire to write a novel of her own, a fact she keeps hidden because she is ashamed of it. Her taste in literature leans toward mysteries with occult or supernatural elements, which is also the genre of the story she wishes to write. Despite her popularity, she seems to be searching for something more meaningful than the superficiality of her everyday school life, and she is drawn to Kasuga when she recognizes in him a genuine and deep-seated love for books.

Aya's role in the story is that of a catalyst for change and healing in Kasuga's life. When they meet, Kasuga is described as being an empty shell of a person, having become withdrawn and aimless after the traumatic psychological events of his middle school years. Her friendship and the simple act of lending him books allow him to rekindle his passion for literature, which was once a core part of his identity. She represents a potential for a healthy, forward-moving relationship, contrasting with the destructive and obsessive connections he had with the two main female figures from his past. Her presence forces him to confront his past and decide what kind of future he wants for himself.

Key relationships define Aya's function in the narrative. Her primary relationship is with Takao Kasuga, who she begins dating after he confesses his feelings for her. This relationship is initially complicated by the fact that she is already seeing someone else when she and Kasuga first become close. More significantly, her relationship with Kasuga is constantly shadowed by his unresolved history with his two former classmates, Sawa Nakamura and Nanako Saeki. Aya is described as having elements of both of these previous characters in a positive way, embodying the qualities that Kasuga sought from each of them, but she is also her own person. She is the first person to whom Kasuga fully confesses the story of his past, and her reaction to this heavy revelation is one of needing time and space to process it, showing a mature and realistic approach to emotional burdens.

Throughout the story, Aya undergoes her own quiet development. While she serves as a source of stability for Kasuga, she is not a static character. She is an aspiring writer, and her engagement with Kasuga, who is also trying to find a way to express himself, likely influences her own artistic ambitions. The relationship they build is not without its struggles, as it forces both of them to confront difficult truths. However, the story concludes on a hopeful note, suggesting that Aya and Kasuga are able to resolve their conflicts, allowing Kasuga to finally move forward and articulate his past experiences and future hopes in his writing. Her primary notable ability is her deep and discerning passion for literature, which allows her to connect with Kasuga on a profound level and recognize the kindred spirit beneath his withdrawn exterior.