Isono, Katsuo

磯野カツオ

Sazae's younger brother, an 11-year-old fifth grader.
He is mischievous and naughty, yet also very talkative, good-natured, and family-oriented. He possesses a strong sense of justice and will not tolerate anyone who bullies Wakame or Tarao, even though his well-meaning actions and small lies often cause trouble in the family.
He loves to play and dislikes schoolwork, but he is very quick-witted and has good powers of action and judgment. This tendency is especially noticeable when he plays pranks or plans something for his own gain. In the anime, he is affable, caring, and a "dependable big brother" to his sister Wakame and nephew Tarao.