Arima Ayumi is the stepmother of Takuya Arima, having married his father, Koudai Arima, about six months before the main events. She serves as a chief supervisor for the Geo Technics Corporation, managing a controversial coastal construction project in Sakaimocho facing public criticism due to unexplained lightning strikes causing fatal incidents among workers. Officially conducting geological surveys, her true assignment involves locating a supernatural mineral called "Psychite" beneath the coastline. She is tasked with crystallizing this mineral into "Jewels" using data from Koudai's research, developing awareness of the full implications over time.

Her relationship with Takuya is complex, marked by grief over Koudai's recent death and struggles to balance corporate obligations with family. Initially pragmatic, she dismisses Takuya's claims about parallel worlds without concrete proof. However, multiple timelines reveal her vulnerability; under extreme stress from project failures and societal pressure, she risks professional ruin or suicide unless Takuya intervenes with the Reflector Device.

Later transported to the parallel world of Dela Grante alongside the antagonist Kōzō, Ayumi assumes the role of Divine Emperor tasked with preventing planetary annihilation. Every 400 years, Dela Grante's orbit nears Earth's, requiring a ritual sacrifice of a living host—later revealed as Yu-no, Takuya's daughter—to power the world's AI system and avert collision. Ayumi orchestrates this plan, believing it necessary for survival, but confronts Takuya attempting to stop the ritual. She reveals her actions were driven by a desire to save both worlds and atone for her past, highlighting her transformation from a corporate manager to a ruler burdened by apocalyptic stakes.

Her character design evolved across adaptations: the original depicted triple-braided hair and a pink suit, while the remake features an updo with a hairpin and a black casual suit.