For starters, Junko and Mukuro are two different people, it's just that they're definitely gonna share a VA, just tradition.  Any who, Junko Enoshima, otherwise known as Super High School Level Despair.  The mastermind of this killing game and basically the embodiment of despair.  She's got a lot going on, so I'll try my best to break it down for you: Junko is a VERY flamboyant character, she is constantly switching personalities due to how easily bored she gets, despite that, she's definitely not stupid.  She has a very cunning mind, being able to analyze everything around her and deduce different outcomes of situations.  Basically, she's a crazy son of a gun.  Mukuro Ikusaba, on the other hand, is Junko sister and her partner in crime.  She poses as Junko for the first chapter of the game until she is killed off.  Mukuro is basically just what Junko is but on less steroids, she is a teenage girl who is into fashion and stuff like that.  This VA should have a mid to high ranged female voice, a real knack for acting dramatic, and strong singing skills.  For the audition, introduce yourself and just say two of the provided lines.  Also, sing a song you think would fit the character, best of luck!