Natsuno Arimura is the second eldest sister in the Arimura family. Following their father's death from grief after their mother Misako vanished during a Paris trip, the four sisters were orphaned, leaving eldest sister Haruka to manage the household.

Natsuno, in her twenties, works as an office lady. Her characterization consistently highlights an intense preoccupation with romantic relationships and marriage prospects, bordering on obsession. This drive significantly influences her actions within the narrative.

Her personal storyline pivots when she encounters the magical dog Junkers, capable of granting three wishes. Upon learning of his power, Natsuno immediately uses one wish to manipulate the romantic feelings of a handsome male coworker who previously rejected her, resulting in his marriage proposal. This act of altering another person's emotions introduces conflict within the family regarding responsible use of Junkers' remaining miracles.

Natsuno joins her sisters traveling to Paris after seeing their mother featured in a television news report. Confronted with Misako's amnesia and her new life caring for another man's daughter, Natsuno participates in the collective family decision. They prioritize the emotional needs of the young French girl lacking a mother over their own desire for reunion, choosing to leave Misako in France without initially restoring her memories via Junkers' final wish. This choice signifies a shift from Natsuno's earlier self-centered motivations towards broader familial responsibility and sacrifice.

Her development culminates in a shared dream sequence with all sisters, where their deceased father commends their mature handling of the difficult decision about their mother. This experience reinforces her integration into the family unit's collective growth and acceptance of their circumstances.