Isanari Uesugi is the father of Fuutarou and Raiha Uesugi. Raising his children alone after his wife died in an accident, he worked as a cameraman, a profession demanding travel and long absences from home. This period involved significant financial struggles, including debts he diligently repaid.

He possesses spiky blond hair and a muscular build, maintaining a notably youthful appearance despite having a high-school-aged son. Years later, he shows only minor wrinkles. He consistently wears sunglasses on his forehead, a keepsake from his late wife who had poor eyesight. His personality is wild and easygoing, traits Fuutarou shared in his younger years; both were academically challenged. Despite this rough exterior, Isanari exhibits warmth, approachability, and a strong sense of responsibility towards his family.

Key relationships define his role. He shares a long-standing friendship with Maruo Nakano, dating back to their school days where they humorously called each other "bad friends." This connection directly secured Fuutarou’s tutoring position with the Nakano quintuplets. Isanari also knew Rena Nakano, Maruo’s late wife, who was his teacher and a link between the two men. He discreetly supported Itsuki Nakano during her encounter with her biological father, Jinnosuke Mudou, aware of their familial tie.

His influence extends to pivotal plot elements. Beyond arranging Fuutarou’s tutoring job, he offered guidance during critical moments, such as advising Fuutarou on matters of the heart. His past actions, like lending his camera for a school trip to Kyoto, subtly impacted character dynamics. His bond with Maruo and history with Rena underscore his role as a connective figure between the Uesugi and Nakano families.