A 10-year-old girl. Satsuki is Mei's older sister. Satsuki is the traditional name of the fifth month of the Japanese calendar, the equivalent of the English May.
As they wait, Mei eventually falls asleep on Satsuki's back and Totoro appears beside them, allowing Satsuki to see him for the first time. He only has a leaf on his head for protection against the rain, so Satsuki offers him the umbrella she had taken along for her father. In return he gives her a bundle of nuts and seeds.