Isanari Uesugi is the father of Fuutarou and Raiha Uesugi. He became a single parent after his wife died in an accident, a tragedy that left him to raise his two young children entirely on his own. During this period, he worked as a cameraman, a profession that often required him to travel and be away from home for extended periods while he worked to support his family. The family experienced significant financial hardship, including debts which he diligently worked to repay.

Physically, Isanari is a tall man with a muscular build and distinctive spiky blond hair, possessing a notably youthful appearance that belies his age as the father of a high school student. He is almost always seen wearing a pair of sunglasses perched on his forehead. This is not merely a fashion choice but a sentimental keepsake, as the sunglasses originally belonged to his late wife who had poor eyesight. His personality is wild and easygoing. Despite a rough exterior that might suggest a rebellious nature, he is a warm, approachable, and big-hearted individual with a strong sense of responsibility toward his family. His character provides a contrast to his son, as it is mentioned that Fuutarou shared his father's wild and academically challenged personality when he was younger.

Isanari's most significant role in the story comes from his past connections. He shares a long-standing friendship with Maruo Nakano, the stepfather of the Nakano quintuplets, which dates back to their school days where they humorously referred to each other as "bad friends". This friendship is the direct reason Fuutarou is offered the high-paying tutoring job for the Nakano sisters, an opportunity that serves as the catalyst for the entire plot. Furthermore, Isanari knew Rena Nakano, Maruo's late wife and the quintuplets' mother, who was his teacher. This shared history makes him a connective figure between the Uesugi and Nakano families.

As a father, Isanari offers quiet guidance during critical moments. He provides Fuutarou with thoughtful advice on matters of the heart when his son is struggling with his feelings. He is also discreetly protective, as evidenced by his quiet support of Itsuki Nakano during her difficult encounter with her biological father, Jinnosuke Mudou, as he was aware of their familial connection. His past actions also have a subtle impact on character dynamics, such as when his camera was used (or borrowed) by Fuutarou for a school trip to Kyoto, an event that holds significant personal meaning for the protagonist. In terms of abilities, beyond his professional skill as a cameraman, his most notable attribute is his unwavering dedication as a provider and protector who maintains his spirited nature despite the hardships he has faced.