Sanae Maruo (丸尾早苗) is Eiichiro's mother. She is the one who suggested him to go to STC when he wanted to get some exercise to keep himself fit.
Eiichiro's mother has a strong personality and has her say in all the family matters. She is proud of her son and wants to help him. She forced Eiichiro to go to STC, when he claimed he needed some exercise. But she was against him taking it too seriously and was angry when he told her that he wants to become a professional tennis player. Later on she agrees on a condition that if he loses at All Japan tournament, he will quit.