Bertille Althusser  

, ベルティーユ・アルチュセール

A high-school senior had an encounter with Mio due to Sakuras (Takahiros horse) behavior which caused embarrassment in front of two underclassmen. This led to a challenge between Mio and her, resulting in Mio gaining respect for her. Despite being easily embarrassed by things like wearing her school swimsuit, she was bold enough to wear a thong during the joust. Bertille comes from a wealthy family and owns several mansions near the school.