The title character, Maruko, is a nine-year-old third-grade student. She is lazy, disorganized and usually late for school, in strong contrast with her neat and tidy older sister who must share her room with her. Maruko, like many kids, tries to avoid homework and chores, takes advantage of her doting grandfather and squabbles with her sister. Nevertheless, she is a well-meaning child who tries to do good.
Other characters include Maruko's classmate and best friend Tama-chan. A fan favorite is Maruko's kind but absent-minded grandfather. Naive and easily tricked, he keeps a pet turtle and loves watching TV with Maruko. When feeling distressed or nostalgic, he spontaneously retreats to a surreal inner world for a few seconds to improvise a sad yet comical haiku about his state of mind.