Katsuo Isono

Sazae's younger brother who is an 11 year-old fifth grader.
He is mischievous and naughty, but on the other hand, is also very talkative, good-natured, and family-oriented. He also has 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.

(Source: Wikipedia)