Junko Enoshima is Hitler. Worse than Hitler. She is the big bad of this series. Literally one of the most evil characters I've ever seen in my life.
The plot is that she manipulated humanity into a collective hysteria that made people all kill each other and caused the apocalypse. Then this school locked its most talented students in to protect them, but Junko infiltrated it, killed the school staff, erased the students memories and is now making us kill each other so she can livestream it to the world and make a point about how humanity sucks.
However, the Junko we are seeing in the setup might not be Junko. In the games, she is Mukuro, her twin sister. Who is also evil. The real Junko is in a secret room controlling Monokuma. So it's possible the real Junko is just Monokuma role. A reason I believe that is the Junko role only has a roleblock, and Mukuro is the Ultimate Soldier so it makes sense her ability is to beat people. Though sprityo doesn't necessarily have to be 100% loyal to the flavor, so maybe Mukuro (Junko Role) is a good guy or something.
Kyoko is the Ultimate Detective. She is probably the cop. Protect her.
Makoto is that cliche anime hero who shouts things about hope and friendship and magically fixes everything. In this universe (where the villain's powers are based on inducing despair) that's very OP. He's probably the town's main role. Protect him too.
And Toko has a split personality. The other girl is a serial killer. In the games she never actually kills anyone, but here she is likely a serial killer or a vigilante. We should try to figure it out.
Everyone else is just normal teenagers I believe. Assuming sprityo was loyal to flavor.