Futaro Uesugi is the protagonist of the series, a high school student who serves as the private tutor for the five Nakano sisters. He is a top-ranking student at Asahiyama High School, known for his exceptional academic performance that has earned him the reputation as the undisputed academic ace of his grade. Despite his brilliance in his studies, his family lives in difficult financial circumstances, grappling with significant debts, and it is this economic pressure that drives him to seek employment. This need to support his household, combined with a desire to prove his abilities, leads him to accept the job of tutoring the quintuplets.

In terms of personality, Futaro is characterized as diligent, determined, and highly intelligent. However, his relentless focus on academic achievement has come at the cost of his social skills. He struggles significantly with interpersonal interactions and often comes across as blunt, tactless, and cold to those around him. This sharp-tongued exterior initially serves to keep others at a distance, but it masks a deeply responsible and hardworking nature. He is also described as frugal and budget-conscious, a necessity given his family's financial situation. A core part of his internal drive is a wish to become a person who is needed by everyone, striving to prove his worth and value to others.

Futaro's role in the story is fundamentally that of a catalyst for change. He is hired as a private tutor with the seemingly impossible mission of guiding the five sisters to graduate successfully. He begins this task with a strictly academic focus, but his presence inevitably draws him into their personal lives. His key relationships are, of course, with each of the five Nakano sisters, though he interacts with each one differently. With Ichika, he shares moments of quiet understanding, including instances of her supporting him and them getting locked in a toolshed together. His relationship with Nino is the most turbulent, as she initially rejects him openly and acts overprotective of her sisters, only to later undergo a significant change of heart and become one of the most direct in expressing her feelings. Miku is the first sister to openly develop romantic feelings for him, and their relationship is built on consistent progress, studying history together and her efforts to become a better cook. Yotsuba is the first sister to regularly attend his study sessions without being forced, and her constant, cheerful support includes key moments like returning a lost test to him and tending to him when he is sick. Finally, with Itsuki, the first sister to confront him, their relationship is often tied to the main plot, such as discussing his past, his motivations, and her own dream of becoming a teacher.

Throughout the narrative, Futaro undergoes significant personal development. He starts as an isolated and financially struggling student who keeps others at arm's length. Through the persistent effort of tutoring the five unique and troublesome sisters, he begins to lower his defenses. As he dedicates himself to their academic success, he finds his own worldview challenged, and he slowly develops meaningful bonds and deeper feelings for them. He evolves from seeing the tutoring job as a mere financial necessity to genuinely caring for the sisters' futures. His most notable ability is, without a doubt, his incredible academic prowess, which allows him to remain at the top of his grade despite his heavy work schedule. However, this talent is paired with the patience and determination required to try and teach five students who start with almost no interest in their studies.