Who is she?
Most of her life is unknown especially in regards to how she became obsessed with despair. Regardless, she and her twin sister Mukuro found themselves in different circumstances such as living homeless for a while. Along the way, Junko became a model and was eventually enrolled in Hope's Peak Academy. Her real skill was "Ultimate Analyst" where she was superhumanly intelligent but this grew to bore her due to her being able to predict the outcomes of events. As such, she became enamored by despair due to its unpredictability and wanted to, in her own words, "bone the whole world" in despair.
From there, she began to plot. She entices Izuru Kamukura into assisting her in her plans of "Ultimate Despair" after realizing that he was similar to her. This segues to the Student Council Killing Game which Junko shifts the complete blame on Izuru. She then causes the Reserve Students to stage a riot when she leaks footage of the game. A skilled corrupter, Junko manipulates other students to her cause and prevents five students from killing themselves mostly out of self-interest since she could get Monokuma robots mass produced through Monaca. She also learns how to use brainwashing technology as a means of enhancing the footage of the Killing Game to make a "Despair Video."
More insanity ensues with Junko convincing Monaca and the other students to commit more despair by claiming that all the adults were demons to be kille. Riots started to leak through Hope Peak Academy's walls and into the world where violence and wars raged until eventually the whole world fell into despair. At Junko's behest, the Reserve Students commit mass suicide. This was the birth of The Tragedy, or The Biggest, Most Awful, Most Tragic Event in Human History, or, alternatively, The Worst, Most Despair-inducing Incident in the History of Mankind. We'll just refer to it as The Tragedy for convenience.
Personality
Oh boy. She is definitely insane and unpredictable. Her "true" personality is that of a cold, embittered madwoman who gets bored with little effort in part due to her abilities as the "Ultimate Analyst." As such, Junko has the tendency of switching personalities on a dime. In her own profession, she has no real reason behind any of the atrocities she commits and does everything out of a sadistic delight.
Despair is her defining characteristic as she is obsessed with it...so much so, you can say that she physically gets off to it. As mentioned, she loves despair because of its unpredictability and wants to bathe the entire world in that feeling. It doesn't matter whether she wreaks despair on others or herself as long as she got her fix. She is also extremely intelligent and manipulative.
But what is interesting to note is that she isn't a true "sociopath." While she tacks off most of the boxes for the criteria (manipulative; superficial charm; lack of remorse or moral convictions), she is able to give affection to people. She claims to have loved her sister; Matsuda; her classmates. However, this only fuels her lust for despair. Junko ultimately views them all as objects that she will form relationships with solely so she could indulge in despair when she kills them. Hence is why she murders Mukuro and Matsuda on a whim: she is legitimately thrilled to further fuel her fetish.
Even when she is defeated and to be excecuted, Junko basks in the delight of feeling her plans crumble around her and cheerfully offs herself despite intially Makoto trying to talk her out of it.
Abilities
Manipulation is her forte having corrupted several students. Even just being in close proximity to her drives people insane. With her talent of being the "Ultimate Analyst," Junko is able to analyze her surroundings at near lightning speed even if she sees something once and memorizes it. This is best exemplified when she uses her brainwashing technology for her Despair Video and later create an AI of herself. This plays into her skills at manipulating others: reading them before they make their move.
Appearance
Tall, hourglass figure (oh yeah); strawberry hair in long pigtails. She has light blue eyes and long red fingernails (whether they're genuine or fake is up to you). A black cardigan covers her white shirt with a button deliberately undone to expose her cleavage. However, her defining traits are her Monokuma hair pieces: on the left, white; the right, black with red eyes and a twisted smile. Other than that, a red miniskirt and different items pertaining to her Former High School Life.
Verdict
8/10