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. However, she is capable of maintaining one persona for extended periods of time when necessary, such as when she was overseeing the students through Monokuma. Junko's "moods" included (but were not limited to) her Monokuma persona, sickening cuteness, severe depression, clinical sophistication, boisterous vulgarity, eerie stoicism, and haughty superiority. She switched between "moods" regularly, often imitating other students' expressions, which made for a highly unsettling conversation with the survivors. While entering the Hope's Peak Academy as one of Class 78's students, she often talked as if narrating a story, writing down her thoughts in a notebook. 

 Above all, Junko was deeply obsessed with despair, having found despair to be the most (and possibly the only) enjoyable emotion for her, whether it be inflicting despair unto others, or suffering from despair wholeheartedly. This made her extremely dangerous, as she wished to make the whole world feel absolute despair. However, this also often proved to be her biggest weakness, as she gave her opponents a "fair" chance to defeat her in order to make their despair all the greater if they fail, and happily welcomed her own despair if her plans fail.