Ayumi Hizuki is a central character in Shin Ringetsu, portrayed as a third-year student at Akizuki Academy. She shares a classroom with Naoto Rindo, a young man who transfers into her school under unique and pressing circumstances. Ayumi is described as having long, straight brown hair and brown eyes. Her typical attire includes her school uniform, sweaters, T-shirts, pajamas, knee-high socks, and sport bloomers, reflecting her life as both a student and a homemaker.

Ayumi possesses a calm and homely demeanor, which makes her quite popular among her peers even though she is not an assertive or natural leader. She is academically gifted, having received an A-level recommendation for university admission. However, her personal life is heavily constrained by family responsibilities. Following the marriage of her older sister, Yuiko, Ayumi has taken on all domestic chores for the Hizuki household. This leaves her with very little free time for socializing, and despite her popularity, she has not been able to pursue a romantic relationship, nor does she show a strong personal eagerness to find a boyfriend.

The core of Ayumi's role in the story is tied to a curse placed upon the Rindo family, which prevents them from bearing male heirs. To counter this curse, every third generation must take a bride from the Hizuki clan, a family known for its spiritual power. Naoto Rindo, the heir, is thus compelled to conceive a child with a Hizuki woman. His intended fiancée was Ayumi's sister, Suzune Hizuki. However, Suzune falls into a comatose state following a car accident, with her soul becoming trapped inside a toy ring she had received from Naoto in childhood. This ring becomes the conduit for a supernatural connection between Ayumi and Naoto.

As the story progresses, Ayumi becomes the unwitting host for her sister’s spirit, which occasionally possesses her through the ring. To revive Suzune, Naoto and Ayumi are driven to engage in repeated intimate encounters, with the goal of conception serving as a necessary component of the ritual. Through this process, the initially shy and duty-bound Ayumi develops genuine romantic feelings for Naoto. This creates a profound internal conflict for her: she must wrestle with her own emerging love and loyalty to her sister, whose spirit is still present. Her eventual pregnancy with Naoto's child deepens this turmoil, complicating the original mission to break the Rindo curse and forcing her to decide whether to step aside for Suzune or fight for her own relationship.

Ayumi’s key relationships are defined by her family and her connection to Naoto. She has several sisters, including the maternal Yuiko, the dominant and sharp-tongued Shino who is a teacher at her school, and the younger Mizuna. However, the most pivotal relationship is with Suzune, whose absence and lingering presence drive the plot. Ayumi’s dynamic with Naoto evolves from one of obligation to genuine affection, forming a complex love triangle with the memory and spirit of her sister. Over the course of the narrative, Ayumi develops from a calm, self-sacrificing homemaker into a woman who must confront her own desires and jealousies, ultimately making significant choices about her future and her family. She does not possess overt supernatural combat abilities, but her spiritual heritage and her role as a vessel for her sister’s soul are central to her function in the story.