Futaba Watanuki, 23 at the series start, is the middle sister residing in the inherited family home in Tokyo's Monzen-Nakacho district with her older sister Ichika and younger sister Mitsuki. Both parents are deceased: their mother died during the sisters' early childhood, and their father passed away approximately one year before the story begins. A high school graduate, Futaba works as a clerk in an apparel shop, reflecting her fashion interest. Outwardly energetic and fashionable, she possesses traditional skills including expertise in calligraphy and shogi. Her personality is hotheaded and occasionally sloppy, yet she has strong communication abilities, making her the most socially adept sister. She holds the most romantic experience in the family, leading her sisters to seek her advice on love. Physically, she has light auburn hair and grey eyes.

Across the series, her family role evolves from a sibling focused on her own pursuits to one increasingly relied upon for emotional and practical guidance, particularly concerning interpersonal relationships. Her development is shown through interactions and the responsibilities she undertakes to maintain household harmony amid everyday challenges. In the second season, her narrative continues within an expanded social environment, though her core role as a supportive sister and her family relationships remain central to her character arc.