Fūtarō Uesugi was born on April 15. After his mother's death when he was six, he grew up with his father, Isanari Uesugi, and younger sister, Raiha Uesugi. Persistent financial difficulties forced the family into a frugal lifestyle, running a small cafe beneath their apartment. Limited means meant Raiha handled all household cooking, resulting in Fūtarō developing an inability to distinguish between well-cooked and poorly prepared food. His financial situation remained largely unchanged even by his wedding day.

Five years before becoming a tutor, a school trip to Kyoto brought Fūtarō into contact with a girl who profoundly influenced his outlook. Inspired to become someone "needed" by others, especially to support Raiha, he dedicated himself to intense study. He did not learn the girl's name then, though she was later revealed as Yotsuba Nakano, one of the quintuplets he would tutor. Academically, Fūtarō excelled, ranking at the top of his class at Asahiyama High School and earning perfect scores, including 100 in all five subjects during his second-year midterms. His strengths lay in exceptional memory, reading comprehension, and the ability to absorb dozens of books within days. He acknowledged weaknesses in social interactions and technology, initially having only his father and sister in his phone contacts.

Physically, Fūtarō has fair skin and bluish-black hair styled in a bowl cut with two strands protruding upward, a haircut he attributed to Raiha. He often slouched despite his tall build and maintained a stern facial expression described as "scary" or resembling a "bad guy." His standard attire was his school uniform—a short-sleeved white shirt, dark blue trousers, and a white belt—supplemented by a light blue V-neck sweater in colder weather. Childhood photos showed him with spiky blonde hair and an earring, resembling his later disguise as "Kintarō," where he wore a blonde wig, black shirt, and white trousers.

Initially, Fūtarō displayed a blunt, antisocial personality, viewing romance as a distraction from academics. He was tactless, egotistical, and avoided involvement in others' problems. His father noted he had once been wild and rebellious, similar to his own youthful demeanor. Tutoring the Nakano quintuplets fostered patience and emotional insight. He initially took the job for financial reasons but grew genuinely committed to their academic success and personal growth, stating his goal was to ensure all five graduated "with a smile." This dedication extended beyond academics, as he supported them through personal challenges, such as helping Nino confront her resistance to change.

His relationship with the quintuplets evolved significantly. Yotsuba was the first to accept him as a tutor, becoming his confidante. Miku developed feelings early, prompting her to improve her cooking after learning his preferences. Nino initially antagonized him, even sabotaging his efforts, but later became openly affectionate after recognizing his sincerity. Ichika grappled with her feelings, sometimes resorting to underhanded tactics before ultimately supporting his choices. Itsuki maintained a platonic bond, often discussing familial or aspirational topics. Fūtarō eventually married Yotsuba, whose identity as the Kyoto girl was revealed later. Their honeymoon in Hawaii included all quintuplets, where Fūtarō coordinated efforts to help a local girl named Lily.

Fūtarō possessed niche skills like sleeping upright with his eyes open and riding a motorcycle, a necessity for a past delivery job. He believed in charms and horoscopes and favored curry udon. Conversely, he had low physical stamina and strength, struggling to piggyback Miku, and was inept at cooking. In spin-offs like the fantasy RPG "The Quintessential Princesses," he appeared as a magic academy student tutoring noble quintuplets.