Futaba Marui is the middle child of the three Marui triplets who are the central characters of the series. She is a sixth-grade elementary school student distinguished by her boundless energy and straightforward, uncomplicated nature. Her most immediately noticeable physical trait is that she is almost always barefoot, a quirk that goes unremarked upon by other characters in her world. Her typical hairstyle involves a single ponytail on top of her head, though the appearance of a more elaborate three-part style is also common.

The defining aspect of Futaba’s character is her extraordinary physical strength, which is presented as being far beyond that of a normal girl her age. She possesses immense athletic prowess, capable of feats such as breaking classroom doors and desks with her bare hands, bending metal chair legs, and knocking over adults with a light kick. Her athletic ability is particularly dominant during sports competitions, where her power and motor skills make her an almost invincible competitor. Despite her immense power, she has very little control over it, leading to frequent accidental destruction of school property and unintended injuries to her sisters. This combination of superhuman strength and a lack of restraint creates much of the physical comedy associated with her.

In contrast to her brute strength, Futaba is intellectually simple and academically struggles. She has notable difficulties with basic subjects, including writing and arithmetic. Her mind operates in a pure, literal, and uncomplicated way, which means she often does not grasp complex social situations or underhanded motives. She is described as being childish for her age, evident in her continued and earnest belief in Santa Claus and her genuine fear of ghosts. This lack of guile, however, makes her surprisingly capable of delivering blunt and accurate observations, providing a simple form of common sense that contrasts with the more convoluted schemes of those around her. One of her most recurring and unique characteristics is her open obsession with large breasts, which she approaches with an almost artistic dedication and enthusiasm. She is also the only one of the three sisters who maintains a consistently close and affectionate relationship with her father, Soujirou.

Futaba’s primary motivation stems from a genuine desire to help others and a deep-seated loyalty to those she cares about. She is a kind-hearted girl who always thinks of others before herself, but her twisted common sense and destructive strength mean her good intentions often backfire spectacularly. She is fiercely protective of her family, as seen when she comes to the defense of her sister Mitsuba, and she is the sister who is most openly affectionate with her father. Her motivations are never selfish or malicious; rather, they are the pure, if poorly executed, impulses of a child who wants to see her loved ones happy.

Within the story, Futaba serves as a primary engine of chaos and physical humor, but also as a surprising source of heart. Her role is often to be the well-meaning muscle who inadvertently escalates a situation into disaster. She is a key part of the sibling dynamic, usually acting as the cheerful, strong intermediary between the more domineering eldest sister, Mitsuba, and the gloomy youngest, Hitoha. Her key relationships include her father, for whom she has an uncomplicated adoration, and her classmate and childhood friend, Shinya Satou, whom she affectionately calls Shin-chan. Her friendship with Shinya is characterized by her innocent affection and his frequent embarrassment, hinting at a developing crush that she herself does not fully comprehend. Among the boys in her class, she is well-liked due to her shared interests and straightforward personality.

Her development over the course of the series is subtle but present. While she remains largely the same muscle-headed optimist, there are indications of slight growth in her awareness, particularly regarding her feelings for Shinya and the consequences of her actions. Her loyal and caring nature also has the chance to shine through more clearly, revealing the sweet and loving person beneath the chaos she creates. Her notable abilities are almost exclusively physical, including her immense strength, high jumping power, speed, and endurance, which allow her to perform feats like running with a heavy adult on her back or crossing a river in a single leap.