Fūtarō Uesugi is a high school student who serves as the main protagonist of his story. He is born on April 15 and stands 178 centimeters tall. Fūtarō lives a frugal life in a small apartment with his father, Isanari Uesugi, and his younger sister, Raiha Uesugi. His mother passed away in a car accident when he was six years old, leaving the family with significant debt and forcing them to run a small café beneath their apartment to survive. Because Raiha handles most of the household cooking with simple and inexpensive ingredients, Fūtarō has developed an inability to distinguish between well-prepared and poorly cooked food, a trait that persists into his adult years.

Five years before the main events of the story, Fūtarō was a very different person. He was a rebellious child with dyed blond hair, an ear cuff, and a wild personality similar to his father's youthful demeanor. During a school trip to Kyoto in sixth grade, he met a girl who profoundly inspired him. She encouraged him to become someone who could be "needed" by others, especially to support his beloved sister Raiha. This encounter motivated him to abandon his delinquent ways and dedicate his life to intense study. He keeps a photograph of this girl, whose face he could not clearly remember because she was one of identical quintuplets, in his student handbook as a lifelong source of motivation.

Academically, Fūtarō is a prodigy. He is consistently at the top of his grade at Asahiyama High School, famously earning perfect scores on every test. His abilities include an exceptional memory, high reading comprehension, and the capacity to absorb dozens of books in just a few days. He acknowledges that his forte lies solely in studying, and he takes great pride in his intellectual capabilities. However, this academic obsession has come at a cost. Fūtarō is socially isolated, blunt, and often tactless in his communication with others. He has a serious expression that can appear unfriendly or annoyed, and his initial approach to problems is to be direct and harsh, which frequently upsets people. He has no friends at the start of the story, with only his father and sister saved in his phone contacts. He views romantic relationships as a foolish distraction from his academic goals and has no experience with love, often reacting with confusion or discomfort when confronted with romantic feelings.

His personality is defined by a combination of stubbornness, high confidence, and a strong sense of duty. He is very hardworking and possesses immense patience, which becomes essential when he accepts a job as a private tutor for the five beautiful but academically struggling Nakano quintuplets. He initially takes the position solely for the high pay to help his family's finances. Despite the sisters' initial hostility towards him, his persistence, honesty, and genuine dedication to their success gradually earn their trust. He moves from a purely transactional relationship to developing deep emotional bonds with each sister. He learns to understand their individual problems, adapt his teaching methods to their unique personalities, and support them through personal challenges that go far beyond academics. His initial goal evolves from simply improving their test scores to ensuring that all five of them can graduate "with a smile."

Fūtarō's key relationships are central to his development. His bond with his sister Raiha is one of profound love and protectiveness, as she is his primary motivation for working hard. With the Nakano quintuplets, his role shifts from an outsider and tutor to a trusted confidant and eventually to a romantic partner. He learns to distinguish the identical sisters through subtle cues such as their speech patterns, body language, and even their handwriting. Each sister challenges and changes him in different ways, forcing him to become more empathetic, open-minded, and aware of other people's emotions. He also forms his first friendships with classmates, slowly coming out of his self-imposed isolation.

Throughout the story, Fūtarō experiences significant character development. He gradually sheds his egotistical and antisocial shell, becoming more adept at navigating human relationships. He begins to value emotional intelligence alongside academic intelligence. The boy who once declared love to be a foolish act eventually comes to understand and accept his own romantic feelings, leading him to confess and choose one of the quintuplets as his future bride. Despite this growth, he retains core aspects of his personality, including his serious demeanor, his lack of athletic ability, and his practical, no-nonsense approach to life.

In terms of notable abilities, Fūtarō possesses a genius-level intellect and an almost photographic memory, making him an unparalleled tutor. He is highly observant and can quickly notice when someone is troubled or hiding something. He is also surprisingly resourceful, able to strategize and think on his feet to solve problems. However, he has notable weaknesses. He has very low physical stamina and strength, struggling with tasks like carrying heavy objects or giving someone a piggyback ride. He is also inept at cooking and unfamiliar with modern technology, initially not even owning a smartphone. Regardless of these shortcomings, his defining trait is his incredible work ethic and his ability to persistently push forward for the sake of those he has come to care about.