Althusser, Bertille

ベルティーユ・アルチュセール

She is a high-school senior who had a run-in with Mio. Apparently, this was due to Sakura's (Takahiro's Horse) mischief, which caused her to be embarrassed in front of her two underclassmen and led her to challenge Mio (who was in charge of Sakura at the time) to a joust. She eventually lost and gained respect for Mio.
She is easily embarrassed by being seen in, for example, her school swimsuit, yet brazen enough to wear a thong during a joust.
Coming from a wealthy family, Bertille owns several mansions located near the school.