She is a high-school senior whom Mio had a run in with. Apparently, it was because of Sakura's (Takahiro's Horse) mischief which led to her being embarrassed in front of her two underclassmen and challenge Mio (who was in charge of Sakura at that time) to a joust. She eventually lost and gained respect for Mio.

She is easily embarrassed by being seen in e.g. her school swimsuit, but brazen enough to wear a thong during a joust.

Coming from a wealthy family, Bertille owns several mansions located near the school.

(Source: Zinc-view)