Katsuo Isono is the younger brother of the title character Sazae in the long-running series. He is an eleven-year-old boy in the fifth grade. As the eldest son of Namihei and Fune Isono, he lives in a three-generation household with his parents, his older sister Sazae, her husband Masuo, their son Tarao, and his younger sister Wakame.

Katsuo has a well-known mischievous and naughty streak, often finding himself at the center of trouble or playing pranks. His well-intentioned actions and small lies frequently cause problems for his family. Despite his troublesome side, he is also described as being good-natured, talkative, and very family-oriented. He possesses a strong sense of justice and will not tolerate anyone who bullies his younger sister Wakame or his nephew Tarao. In his role in the family, he acts as a caring and dependable big brother to them.

While Katsuo loves to play and dislikes schoolwork, he is quick-witted and possesses good judgment and decisive action, especially when planning mischief or pursuing his own gain. His hobbies include playing baseball and soccer with his friends. His best friend is a boy named Nakajima, with whom he often plays sports. At school, he sits next to a girl named Hanazawa Hanako, who is notably strong and assertive. He is also the subject of scolding from his older sister Sazae, particularly when he refuses to do his homework or accidentally insults guests.