Ayumi Arima is Takuya Arima's stepmother, having married his father, the historian Koudai Arima, approximately six months preceding the story's events. Her marriage followed Koudai's previous union with Keiko Arima, Takuya's biological mother, who died when Takuya was very young. Following Koudai's disappearance and presumed death, Ayumi became a young widow who coped with grief through work, retaining an underlying sadness. Professionally, she serves as a chief worker at Geo Technics, a company conducting geological surveys and construction at Sakaimachi's coastal Sword Cape, also known as Triangle Mountain. Ayumi bears significant responsibility for the project, facing public scrutiny and media pressure due to unexplained lightning strikes injuring workers. Her role demands defending the company's safety protocols against escalating local opposition. Secretly, the project aims to locate and crystallize a supernatural mineral called "Psychite" or "jewel" beneath the coastline, based on Koudai's research. This mineral is tied to interdimensional travel and parallel world stability. Ayumi's involvement in this hidden agenda creates additional stress as she navigates corporate expectations and morally ambiguous directives from superiors, including her subordinate Hideo Toyotomi, who harbors inappropriate intentions while seeking company secrets.

Her personal life reflects professional burdens. She is a competent homemaker noted for her cooking skills but struggles with severe myopia, rendering her nearly blind without glasses. Her emotional state becomes increasingly fragile due to workplace scandals and unresolved grief, leading to moments of despair. Across multiple timelines explored through branching narratives, this culminates in suicide attempts or completion, particularly when protests mount or her professional failures intensify. Ayumi's relationship with Takuya evolves significantly across different routes. Initially defined by their step-relationship, their connection deepens as Takuya uses the Reflector Device—an interdimensional tool inherited from Koudai—to intervene in her fate. In several paths, Takuya's efforts to prevent her suicides or expose conspiracies lead to emotional confrontations where Ayumi reveals her need for affection and support. This vulnerability fosters mutual romantic feelings, resulting in some branches in a committed relationship and endings where they live together happily.

Her character design differs between media. Original depictions show her in a pink suit with three braids over one shoulder, while later remakes feature her hair gathered with a hairpin and a black casual suit. In the overarching plot, transported to the parallel world of Dela Grante alongside the imposter Kōzō Ryūzouji seeking the Reflector Device, Ayumi ascends to Divine Emperor. In this role, she attempts to prevent Dela Grante's collision with Earth by overseeing a ritual requiring the sacrifice of Yu-no, Takuya's daughter, forcing her into morally complex decisions balancing world preservation against personal connections. Her efforts are thwarted by Kōzō's interference, leading to catastrophic outcomes. Ayumi's ultimate fate varies across branches: some timelines conclude with peace alongside Takuya, while others, particularly involving Dela Grante, result in her death or ambiguous resolution. Her arc emphasizes themes of responsibility, resilience in loss, and the search for emotional anchorage amid intersecting worlds.