As a young fashionista, Junko is described as simply "not fake", charismatic and eager to try new things. During her first appearances, she comes across slightly air-headed and hot-tempered. In the official art book, she is described as a person who "drips with dissatisfaction the moment she opens her mouth" and tends to be in trouble with her classmates. She is also mentioned to be a bit too talkative with an overly carefree attitude. However, Junko's real personality, revealed at the conclusion of the Killing School Life, is notably different: very apathetic and easily bored to an unhealthy and abnormal degree. Because of this, her behavior and mannerisms tended to be extremely erratic and unstable by her own autonomous design; keeping up one "appearance" for too long bores her, which causes her to often switch between different personas.

Voice reference: https://www.youtube.com/watch?v=1uJ1Jg5Pj4U