Fune Isono is the matriarch of the Isono family and the mother of Sazae, Katsuo, and Wakame. She is married to Namihei Isono, the firm but loving head of the household. In the anime, she is depicted as being in her early fifties, while in the original manga, she was consistently forty-eight years old.

As the family's mother, Fune is the embodiment of a traditional, patient, and nurturing homemaker. She is almost always seen wearing a simple, dark-colored kimono topped with a cooking apron, an outfit that has become her visual trademark. Her gentle and kind nature is the emotional core of the household, and she possesses a warm, steadfast demeanor that allows her to calmly observe and care for her family as they go about their daily lives. All members of the family hold her in high regard and instinctively respect her quiet authority, which contrasts with her husband Namihei's more vocal and quick-tempered personality. Despite her calm exterior, she has a hidden mischievous side; rumors suggest she was a bit of a tomboy in her youth, and even as an adult, she enjoys playfully teasing her husband, showing the strong and affectionate bond they share.

Fune's primary motivation is the happiness and stability of her family. Her role in the story is to serve as the steady, reliable foundation upon which the often-chaotic life of the Isono household rests. She is the classic "good wife and wise mother" (ryōsai kenbo), excelling at all manners of housework, with her cooking being particularly notable. She dedicates herself fully to her domestic duties, working diligently from early in the morning to care for her husband, children, and grandson, Tarao. Her nurturing nature is so intrinsic to her character that she feels anxious when told to rest or take time for herself. While she rarely raises her voice, she becomes a formidable and strict presence when her children behave rudely or bring shame to the family; her calm scolding can be so powerful that even Namihei dares not oppose her when she is truly angry.

Her key relationships define her character. With her husband, Namihei, she acts as a calming counterbalance to his grumpy and sometimes rash behavior, often managing him with quiet grace and a touch of gentle mockery. She is a devoted and loving mother to Sazae, Katsuo, and Wakame, offering them patient guidance and unconditional support. A memorable early storyline mentioned that she initially worried her daughter Sazae was too unladylike to ever find a husband, highlighting her concern for her children's futures. She also shares a significant friendship with her next-door neighbor, Oukaru Isasaka, who was her classmate from a Christian girls' school; the two maintain their bond from childhood and affectionately refer to each other as "Ofune-chan" and "Okaru-chan".

While the characters of Sazae-san do not age, Fune's depiction has subtly developed in its long history. In the original manga and the very early episodes of the anime, she was portrayed with a much more fiery and strict personality, prone to outbursts and Spartan discipline. Over the decades, her character was refined into the serene, kind, and almost universally patient mother figure known today. This version is a pillar of strength for her family, possessing notable abilities that include a seemingly inexhaustible capacity for hard work and a deep, intuitive skill for managing a household. Despite her competence, she retains small, endearing vulnerabilities, such as being afraid of mice and frogs, which adds to her humanity and charm. A rumor among fans suggests she might be Namihei's second wife due to the age gap between her children, but this is never confirmed or addressed in the official story.