The phrase "main baddie" refers to the primary or most important villain or antagonist in a story, movie, or game. It is the main person or creature that causes trouble or conflict for the main character or hero. Full definition
The plot involves the further machinations of main baddie Jeanine (not a terribly scary villain's name, is it), played with icy smoothness by Kate Winslet. (vanityfair.com)
Or main baddie Doviculus and the way he just delights in the pain you unleash on him in the final boss battle. (waypoint.vice.com)