Aren't these all IdolM@ster characters who were in Grandblue?
Nope. You're thinking of Cinderella Girls. This is Million Live. Million Live just happens to have plenty of audio dramas with idols in villainous roles in them ranging from serial killers to supervillains to just plain old rival idols. Not hard to see why parka would draw them like the edgy Azur Lane images a while back.