Ayumi originated in a past life as a turtle owned by Gorō Mutsumi. Born on November 20, 1984, she was adopted and named Ayumi by Gorō. During her winter hibernation, Gorō dug a hole for her near a ginkgo tree in a park, assisted by a girl sharing the turtle's name; this shared task forged an immediate friendship between them. When the park underwent demolition the following spring, the ginkgo tree sheltering Ayumi was cut down, crushing her to death. Gorō and the girl buried her, an event that later manifested in Ayumi's afterlife as fears of confined spaces and darkness.

Reincarnated as one of twelve guardian angels protecting Gorō, Ayumi holds the position of Senior-rank angel. Her personality reflects her turtle origins, often displaying deliberate and forward-thinking decision-making. She frequently argues with Mika, another angel reincarnated from a rabbit, echoing the tortoise-and-hare dynamic. Ayumi typically wears a green beret-like hat symbolizing her former shell.

Her traumatic death contributes to persistent phobias: claustrophobia (fear of tight spaces) and achluophobia (fear of the dark), presenting ongoing challenges. Beyond her guardian duties, Ayumi is designated as one of the brides for the Four Saint Beasts, specifically paired with Shin, who represents the Black Turtle (Genbu). Among the angels abducted by these beings, Ayumi demonstrates the most genuine reciprocation of feelings toward Shin as their relationship evolves.

Ayumi enjoys drawing and is occasionally depicted sketching. Her preferred color is green. Across the series, her character development involves confronting and gradually overcoming her deep-seated fears, enabling her to fulfill her protective role more effectively and navigate her complex bond with Shin.