Futaba Watanuki is the middle sister among the three Watanuki siblings and a central character in the series. She is twenty-three years old at the start of the story and resides with her older sister, Ichika, and her younger sister, Mitsuki, in the family home located in the Monzen-Nakacho district of Tokyo. The sisters have faced significant loss, as their mother passed away during their early childhood, and their father died approximately one year before the series begins.

In terms of her background and daily life, Futaba is a high school graduate who works as a clerk in an apparel shop, an occupation that reflects her genuine interest in fashion. Her appearance is characterized by light auburn hair and grey eyes. Despite her fashionable and energetic exterior, Futaba has a personality that is often described as hotheaded, having a somewhat coarse or rough demeanor, and she can occasionally be sloppy. However, this exterior belies her true strengths; she possesses exceptionally strong communication skills, which make her the most socially adept and sociable of the three sisters. This social intelligence extends to romantic matters, as she has the most relationship experience in the family, leading her older and younger sisters to frequently seek her advice on love and interpersonal issues.

Futaba also has notable abilities that contrast with her modern, casual job. She demonstrates expertise in traditional Japanese arts, including calligraphy and shogi. These skills add depth to her character, showing a blend of contemporary and traditional facets.

Her role in the story is primarily defined by her position within the family unit. As the sisters navigate their daily lives and cope with the shared loss of their parents, Futaba transitions from being a sibling focused on her own pursuits to a crucial source of emotional and practical support. She is increasingly relied upon to provide guidance, particularly regarding social and romantic challenges, and she undertakes responsibilities to help maintain household harmony amidst the everyday trials they face. Her character development is demonstrated through her interactions with her sisters and the evolving trust they place in her, solidifying her role not just as the passionate middle sister, but as a key pillar of guidance for the Watanuki household.