Miyabi is essentially the leader of the class terrorism operation. She allows herself to be consumed by the terrorism, essentially letting her hatred of teachers control her. This leads her to feel betrayed by the others when they gradually take Onizuka’s side. She ends up distancing herself from her classmates, becoming a loner, dyeing her hair and dressing like a “slut.” She’s also not happy with the situation in her home, where she feels like her parents are pretending to like each-other, when they really seem to hate each-other, leading her further down the path of slut.
She comes off as a bitch for most of the series and it isn’t until the end when her bitchy personality is explained.