Asuka Kaminogi is the mother of the series protagonist, Haruka Kaminogi, and a former resident of the city of Hakodate. At thirty-eight years old, she is a professional writer and illustrator of children's picture books, a career she works on from home. After her divorce from her ex-husband, Takuya Mayuzumi, Asuka made the decision to move back to her hometown of Hakodate with her daughter, seeking a new start.

Her personality is frequently described as aloof and self-willed, with a tendency to be somewhat absent-minded or spaced out. This can manifest in a distracted demeanor; for instance, she has been known not to notice immediately when her daughter has gone missing. As a night owl who often works late, she is typically still asleep by the time Haruka leaves for school in the morning. Despite this distant air, Asuka is a deeply loving and caring mother who places a great deal of trust in Haruka, allowing her a significant amount of independence. She has a short temper and a rather forceful way of expressing her irritation, as she has a habit of throwing glass objects or drinks at people when she is angry, a behavior her ex-husband Takuya experienced firsthand.

Asuka's role in the story is primarily that of a supportive parent and a stabilizing presence in Haruka's everyday life, which is frequently disrupted by interdimensional events. Her own past and relationships add depth to the world of the series. She and Miyuki Gotou, the mother of Haruka's friend Yuu, were best friends since childhood. Their close friendship, however, began to deteriorate two years prior to the main events of the series, following the death of Yuu's maternal grandmother in Sapporo. This rift in their relationship is a significant emotional undercurrent until an incident where Haruka and a younger version of Miyuki visit the past, which ultimately allows Asuka and Miyuki to reaffirm and heal their friendship.

Asuka's personal history is further illuminated through her relationship with Takuya Mayuzumi. She first met him in Tokyo while attending art school, a meeting that occurred by accident when she dropped her drawings on a staircase and he was impressed by what he saw. During her student years, her drawing skills were only mediocre; one of her paintings was famously misinterpreted by both Takuya, who thought it was a puppy, and later by Haruka, who thought it was a fat bear, when the intended subject was a cat. While she does not possess any supernatural combat abilities, her notable skills lie in her creative work as a published author and artist, and her character development centers on the rekindling of an important friendship and her unwavering, if sometimes unorthodox, devotion to her daughter.