Fumio Kuga is the younger brother of the protagonist, Ichiro Kuga, and lives with him and their sister, Machi. After the death of their father, Fumio, along with Machi, is taken care of by their older brother, who works as a professional manga artist to support the family. Fumio is a young child, depicted with black hair that reaches his ears and blue eyes, often seen wearing glasses and shorts.

Personality-wise, Fumio is portrayed as the quiet and reserved sibling, often contrasting with his more spunky and talkative sister, Machi. He is described as a silent sibling who is present and observant, contributing to the supportive family atmosphere. The children’s well-being and happiness are central to the household, and Fumio is frequently seen alongside his brother and sister during daily life and outings, such as when the family plans trips to the zoo.

Fumio’s role in the story is primarily as a key member of the Kuga family unit. The domestic life of the household is a major focus, and Fumio represents the family responsibilities that Ichiro carries. The developing relationship between Ichiro and Shiori Goshiki unfolds within this family context, with Fumio and Machi present for many of the couple’s milestones, including when Ichiro formally introduces Shiori as his fiancée. The children's acceptance of Shiori helps integrate her into their lives, and the group is often depicted functioning together like a small family.

Fumio shares a close bond with his older brother, Ichiro, who works hard to provide for him, and with his sister, Machi, with whom he participates in family activities. For instance, on a rainy day when a trip to the zoo is canceled, Fumio joins the rest of the family in an indoor activity to create a new set of fortune-telling tarot cards. This event highlights how Fumio is integrated into the warm domestic scenes that define the series and shows his quiet participation in the family's bonding moments. There is no information available about any notable abilities possessed by Fumio Kuga.