Excluding her height, Mayumi is described as someone with a well-rounded personality and a feminine side. While it is hard to notice, she cares about others in her own way, so she fits the bill for an "older sister" perfectly. She is generally quite confident in her appearance. Initially, Mayumi acts coy around Tatsuya, just as she always does in the presence of other people. Gradually, she reveals her true nature after acknowledging him. She appears to have slight sadistic or childish tendencies, as shown through the bitter Valentine's chocolate she gave to Tatsuya and Hattori Gyoubushoujo Hanzou. Due to this nature, sometimes others playfully refer her as "Little Devil or Imp". Despite her social standing as the third child of the Saegusa Family where she should have already been used to flattery, Mayumi gets embarrassed every time Tatsuya truly praises her, which is noted as something that makes her look like a perfectly normal high school student and hints at the fact that she may have feelings for him. Mayumi also tends to get upset when Tatsuya compliments other girls when they are alone.